task: added warning when none is called in intervention handler

[goyalpramod]

Why are these changes needed?

The user may be returning none by mistake, explicitly informing them about it can be helpful

I have used the Python in-built library warnings to fix this issue, it checks if a handler call is None, if it is. It returns a warning otherwise returns the original result

Related issue number

Closes #3621

Note

I ran all tests and ran into some issues with autogen-core\tests\execution\test_user_defined_functions.py. I am unsure if it was caused by my changes, but I believe that should not be the case.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[goyalpramod]

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[jackgerrits]

@goyalpramod would you be able to fix the two failing checks?

You can find instructions here https://github.com/microsoft/autogen/tree/main/python#development