Add a warning when returning an object w/ num_returns=0

[justinrmiller]

Why are these changes needed?

If you set num_returns=0, it is likely a mistake if you return a non-None object. We should print a warning message in this case to avoid confusion.

This code prints a warning message if num_returns equals zero and the returned object is non-None.

Related issue number

Closes #51789

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