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]

Please name this with a leading underscore to indicate private

[jackgerrits]

IMO there's no reason to return the value, and by doing so you're stripping the type away from it since you're using Any. I'd say best off to just check and warn and return as normal like before