Locating the next sibling content if the existing node is empty - xslt-1.0

Question

Here is a snippet of XML:
<?xml version="1.0" encoding="utf-8"?>
<AssignmentHistory Version="171804">
<W20160229>
<ReviewQuestion>Why will God’s Kingdom have to crush the earthly rulerships depicted in the image? (Da 2:44)</ReviewQuestion>
<StudentItems>
<Item>
<Name Counsel="9" NextCounsel="0" Completed="1">Finlay Truckle</Name>
<Type>Bible Reading (Main)</Type>
<Description>Bible Reading</Description>
</Item>
<Item>
<Name Counsel="38" NextCounsel="0" Completed="1">Name</Name>
<Type>#1 Student (Main)</Type>
<Description>Initial Call</Description>
</Item>
<Item>
<Name>Name</Name>
<Type>Assistant</Type>
<Description>Initial Call</Description>
</Item>
<Item>
<Name Counsel="41" NextCounsel="0" Completed="1">Name</Name>
<Type>#2 Student (Main)</Type>
<Description>Return Visit</Description>
</Item>
<Item>
<Name>¬DELETED¬</Name>
<Type>Assistant</Type>
<Description>Return Visit</Description>
</Item>
<Item>
<Name Counsel="45" NextCounsel="0" Completed="1">Name</Name>
<Type>#3 Student (Main)</Type>
<Description>Bible Study</Description>
</Item>
<Item>
<Name>Name</Name>
<Type>Assistant</Type>
<Description>Bible Study</Description>
</Item>
</StudentItems>
</W20160229>
<W20160404/>
<W20160411>
<ReviewQuestion>What did the immense tree in Nebuchadnezzar’s dream represent? (Da 4:10, 11, 20-22)</ReviewQuestion>
<StudentItems>
<Item>
<Name Counsel="11" NextCounsel="0" Completed="1">Name</Name>
<Type>Bible Reading (Main)</Type>
<Description>Bible Reading</Description>
</Item>
<Item>
<Name Counsel="0" NextCounsel="0" Completed="1">Name</Name>
<Type>#1 Student (Main)</Type>
<Description>Initial Call</Description>
</Item>
<Item>
<Name>Name</Name>
<Type>Assistant</Type>
<Description>Initial Call</Description>
</Item>
<Item>
<Name Counsel="37" NextCounsel="0" Completed="1">Name</Name>
<Type>#2 Student (Main)</Type>
<Description>Return Visit</Description>
</Item>
<Item>
<Name>Name</Name>
<Type>Assistant</Type>
<Description>Return Visit</Description>
</Item>
<Item>
<Name Counsel="0" NextCounsel="0" Completed="1">Name</Name>
<Type>#3 Student (Main)</Type>
<Description>Bible Study</Description>
</Item>
<Item>
<Name>Name</Name>
<Type>Assistant</Type>
<Description>Bible Study</Description>
</Item>
</StudentItems>
</W20160411>
</AssignmentHistory>
Now, in my XSL script I link into the above document like this (just a snippet for now):
<tr>
<td class="cellComments" colspan="4">
<xsl:variable name="AssignHistory" select="document('AssignHistory.xml')"/>
<xsl:variable name="week" select="Date/#NextWeek"/>
<xsl:variable name="NextReviewQuestion" select="$AssignHistory/AssignmentHistory/*[name()=$week]/ReviewQuestion"/>
<xsl:if test="normalize-space($NextReviewQuestion) != ''">
<span class="textReviewQuestionLabel">
<xsl:value-of select="//Labels/NextReviewQuestion"/> 
</span>
<span class="textReviewQuestion">
<xsl:value-of select="$NextReviewQuestion"/>
</span>
<br />
</xsl:if>
<br />
<br />
<br />
<span style="font-size: 8pt;">
<xsl:apply-templates select="$AssignHistory/AssignmentHistory/*[name()=$week]/StudentItems">
<xsl:with-param name="MainHall" select="//Labels/MainHall"/>
<xsl:with-param name="AuxClass1" select="//Labels/AuxClass1"/>
<xsl:with-param name="AuxClass2" select="//Labels/AuxClass2"/>
</xsl:apply-templates>
</span>
</td>
</tr>
Now this is the issue. See the week: <W20160404/>? This week was a special event. We didn't have a meeting because we all went somewhere else for a assembly. As a result, there is no details.
So this is what I want to do:
If the week that I am trying to get details from is empty (special event) attempt to get the value from the next sibling instead (if there is one). Otherwise, if it is not empty (a normal meeting) just use the returned value like I do now.
It does get a bit more complicated for the students bit of script. But the principle is the same. To use the following sibling if required.
I am sure I have worded this much more complicated than it needs to be.
Update
I will reform the question in due course.
Here is a history snippet:
<?xml version="1.0" encoding="utf-8"?>
<AssignmentHistory Version="171804">
<W20160229>
<ReviewQuestion>Why will God’s Kingdom have to crush the earthly rulerships depicted in the image? (Da 2:44)</ReviewQuestion>
</W20160229>
<W20160404/>
<W20160411>
<ReviewQuestion>What did the immense tree in Nebuchadnezzar’s dream represent? (Da 4:10, 11, 20-22)</ReviewQuestion>
</W20160411>
<W20170803>
<ReviewQuestion>Test question</ReviewQuestion>
</W20170803>
</AssignmentHistory>
Here is the main XML snippet:
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="TestTransform.xsl"?>
<MeetingWorkBook>
<Meeting>
<Date ThisWeek="W20160229" NextWeek="W20160404">Date 1</Date>
</Meeting>
<Meeting>
<Date ThisWeek="W20160404" NextWeek="W20160411">Date 2</Date>
</Meeting>
<Meeting>
<Date ThisWeek="W20160411" NextWeek="W20170803">Date 3</Date>
</Meeting>
</MeetingWorkBook>
Here is the simplified XSL snippet:
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html" indent="yes" version="4.01"
doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
doctype-public="//W3C//DTD XHTML 1.0 Transitional//EN"/>
<xsl:template match="/">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
</head>
<body>
<table>
<xsl:for-each select="MeetingWorkBook/Meeting">
<tr>
<td>
<xsl:value-of select="Date/#ThisWeek"/>
</td>
<td>
<xsl:value-of select="Date/#NextWeek"/>
</td>
<td>
<xsl:variable name="AssignHistory" select="document('TestHist.xml')"/>
<xsl:variable name="week" select="Date/#NextWeek"/>
<xsl:variable name="NextReviewQuestion" select="$AssignHistory/AssignmentHistory/*[name()=$week]/ReviewQuestion"/>
<xsl:if test="normalize-space($NextReviewQuestion) != ''">
<xsl:text>Question: </xsl:text>
<xsl:value-of select="$NextReviewQuestion"/>
</xsl:if>
</td>
</tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
Now, if you view the main XML file in IE you will get:
W20160229 W20160404
W20160404 W20160411 Question: What did the immense tree in Nebuchadnezzar’s dream represent? (Da 4:10, 11, 20-22)
W20160411 W20170803 Question: Test question
This shows the problem. The dates are fictional. So this is the context where I want to use the following sibling "ReviewQuestion" if the first attempt returns an empty node.
Update 2
Based on your excellent revision to your answer I was able to make the simpler bit of my script work as expected. I broke down the code a bit more as I needed a conditional label prefix "Question: ". So at the moment I have:
<xsl:template match="/">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
</head>
<body>
<table>
<xsl:for-each select="MeetingWorkBook/Meeting">
<tr>
<td>
<xsl:value-of select="Date/#ThisWeek"/>
</td>
<td>
<xsl:value-of select="Date/#NextWeek"/>
</td>
<td>
<xsl:variable name="AssignHistory" select="document('TestHist.xml')"/>
<xsl:variable name="week" select="Date/#NextWeek"/>
<xsl:variable name="history-week" select="$AssignHistory/AssignmentHistory/*[name()=$week]"/>
<xsl:variable name="NextReviewQuestion" select="($history-week | $history-week/following-sibling::*)/ReviewQuestion"/>
<xsl:if test="normalize-space($NextReviewQuestion) != ''">
<xsl:text>Question: </xsl:text>
<xsl:value-of select="$NextReviewQuestion"/>
</xsl:if>
</td>
</tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
My only remaining bit to address now is that I originally also had this code:
<xsl:apply-templates select="$AssignHistory/AssignmentHistory/*[name()=$week]/StudentItems">
<xsl:with-param name="MainHall" select="//Labels/MainHall"/>
<xsl:with-param name="AuxClass1" select="//Labels/AuxClass1"/>
<xsl:with-param name="AuxClass2" select="//Labels/AuxClass2"/>
</xsl:apply-templates>
I need to do the same thing again now. We need to use the following sibling "StudentItems" if the first one returned nothing.
Then we are done.

Answer 1

Consider the following simplified example:
XSLT 1.0
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" omit-xml-declaration="yes" version="1.0" encoding="utf-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="AssignmentHistory">
<table border="1">
<xsl:apply-templates/>
</table>
</xsl:template>
<xsl:template match="*[starts-with(name(), 'W')]">
<tr>
<td>
<xsl:value-of select="name()" />
</td>
<xsl:choose>
<xsl:when test="*">
<xsl:apply-templates/>
</xsl:when>
<xsl:otherwise>
<xsl:apply-templates select="following-sibling::*[*][1]/*"/>
</xsl:otherwise>
</xsl:choose>
</tr>
</xsl:template>
<xsl:template match="ReviewQuestion">
<td>
<xsl:value-of select="." />
</td>
</xsl:template>
<xsl:template match="StudentItems">
<!-- ??? -->
</xsl:template>
</xsl:stylesheet>
When applied to the given XML example, the result (rendered) will be:
Added:
Regarding the problem presented in the Update part of your question:
Try replacing this part:
<td>
<xsl:variable name="AssignHistory" select="document('TestHist.xml')"/>
<xsl:variable name="week" select="Date/#NextWeek"/>
<xsl:variable name="NextReviewQuestion" select="$AssignHistory/AssignmentHistory/*[name()=$week]/ReviewQuestion"/>
<xsl:if test="normalize-space($NextReviewQuestion) != ''">
<xsl:text>Question: </xsl:text>
<xsl:value-of select="$NextReviewQuestion"/>
</xsl:if>
</td>
with:
<td>
<xsl:variable name="AssignHistory" select="document('TestHist.xml')"/>
<xsl:variable name="week" select="Date/#NextWeek"/>
<xsl:variable name="history-week" select="$AssignHistory/AssignmentHistory/*[name()=$week]"/>
<xsl:value-of select="($history-week | $history-week/following-sibling::*)[normalize-space(ReviewQuestion)][1]/ReviewQuestion"/>
</td>