Not using Where statement in a context - sql

Question

Goal:
If you have the input data that is -10 then you should not use the WHERE statement in function.
Problem:
I do not know how to solve it in this context. You have to use WHERE and not WHERE depending on what input data you retrieve
Info:
If you use -10 as a input data then you should retrieve all data based on [dbo].[testing] and it is okay to retrieve data that is null in [dbo].[testing2] in relation to LEFT JOIN.
*The code and its data is a sample from production phase.
Thank you!
CREATE TABLE [dbo].[testing](
[id] [int] NULL,
[value] [varchar](30) NULL,
[category] [int] NULL
) ON [PRIMARY]
CREATE TABLE [dbo].[testing2](
[id] [int] NULL,
[value] [varchar](30) NULL,
[category] [int] NULL,
[test_id] [int] NULL,
[id_type] [int] NOT NULL
) ON [PRIMARY]
CREATE FUNCTION dbo.testt (
#data int
)
RETURNS TABLE
AS
RETURN
(
SELECT
a.[id],
a.[value],
a.[category],
b.[id_type]
FROM [dbo].[testing] a left join [dbo].[testing2] b on a.id = b.[id]
where b.[id_type] = #data
)
INSERT INTO [test].[dbo].[testing] VALUES
(1, '', 2), (2, '', 3), (3, 'a', 2), (4, 'a', 2),
(5, 'b', 2), (6, 'b', 2), (7, 'c', 2), (8, 'c', 2),
(9, 'c', 2), (10, 'c', 2);
INSERT INTO [test].[dbo].[testing2] VALUES
(3, 'a' ,2 ,11 ,1), (4, 'a' ,2 ,11 ,1),
(5, 'a' ,2 ,11 ,0), (6, 'a' ,2 ,11 ,2);
select
s.[id],
s.[value],
s.[category],
s.[id_type]
from dbo.testt(1) s

Answer 1

Have your WHERE clause check if #data is either -10 or matches b.[id_type].
WHERE (#data = -10) OR (b.[id_type] = #data)

Answer 2

What about where b.[id_type] = #data OR #data = -10 in the testt function ?
So your function would be:
CREATE FUNCTION dbo.testt (
#data int
)
RETURNS TABLE
AS
RETURN
(
SELECT
a.[id],
a.[value],
a.[category],
b.[id_type]
FROM [dbo].[testing] a
LEFT JOIN [dbo].[testing2] b on a.id = b.[id]
WHERE b.[id_type] = #data OR #data = -10
)