Fix Argument 1 to "__call__" of "TranslationFunction" has incompatible type

There is some cases where the `get_description` method will return None.
The TranslationFunction assume the first parameter will be a string, not None.

Alternative solutions:
- update the condition: vtype is not None and != "Any" (picked solution)
- assert returned value is not None whenever necessary
- update TranslationFunction signature to take first parameter as Optional[str].
- update all the `get_description` methods to always return a str.
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