How to raise a exception if the incoming value for a source field is empty

Dear all,

I have a scenario where the source and target structures are as follows :

The output for the above transformation logic is

The occurrence for field id in source and target is 1:1   Even if i don't provide the input value for id field,

system is not throwing any runtime exception.But if the field tag id is missing in incoming payload, then

system throws exception.

1) *** How can i raise a exception it the value for field id is empty

2) As the target structure occurrence for field id is 1:1  why system doesn't throw exception for null value

Regards

Koti Reddy