Mule flatten arrays with one of the arrays as null - mule

Question

I am learning mule4 and while trying out scatter-gather, i have run into an issue.
There are 3 flows in the scatter gather task, 2 of these flows return valid payload with data but the 3rd flow returns a empty payload.
thus when i am trying to flatten these 3 results , i am running into null pointer issue. i am not able to make out way to handle this. can anybody help please.
My dataweave expression is as below
%dw 2.0
output application/java
---
flatten(payload..payload)
logs
ERROR 2018-10-27 07:10:00,249 [[MuleRuntime].cpuLight.06: [apdev-flights-ws].getAllFlightsFlow.CPU_LITE #5abc6c4c] [event: ] org.mule.runtime.core.internal.exception.OnErrorPropagateHandler:
********************************************************************************
Message : "Unexpected end-of-input at payload#[1:1] (line:column), expected false or true or null or {...} or [...] or number but was , while reading `payload`as Json.
1|
^" evaluating expression: "%dw 2.0
output application/java
---
flatten(payload..payload) ".
Error type : MULE:EXPRESSION
Element : getAllFlightsFlow/processors/2 # apdev-flights-ws:implementation.xml:102 (Flatten)
Element XML : <ee:transform doc:name="Flatten" doc:id="46245fd1-5ebd-4b16-b83e-31ea4dbb53e8">
<ee:message>
<ee:set-payload>%dw 2.0
output application/java
---
flatten(payload..payload)</ee:set-payload>
</ee:message>
</ee:transform>
(set debug level logging or '-Dmule.verbose.exceptions=true' for everything)
********************************************************************************
INFO 2018-10-27 07:10:00,323 [[MuleRuntime].cpuLight.06: [apdev-flights-ws].getAllFlightsFlow.CPU_LITE #5abc6c4c] [event: ] org.mule.runtime.core.internal.context.notification.Policy: NotificationListener com.mulesoft.mule.debugger.server.DebuggerPipelineMessageNotificationListener#20d9c50f was unable to fire notification PipelineMessageNotification{action=pipeline process complete, resourceId=getAllFlightsFlow, serverId=de-PC..apdev-flights-ws, timestamp=1540604400323} due to an exception: java.lang.NullPointerException.

Answer 1

I believe it's the payload..payload call that's messing you up. If you want to flatten the payload, it should just be:
%dw 2.0
output application/java
---
flatten(payload)