Fix autodoc IndexError on class with custom __module__ on Python 3.14

[joshuaswanson]

Fixes #14345.

On Python 3.14, inspect.getsource on a class with a custom __module__ can return "\n" instead of raising OSError. ast.parse("\n") then produces an empty module body, and module.body[0] raises IndexError. Added IndexError to the existing except clause in get_type_comment so it returns None gracefully, matching the existing behavior for OSError and TypeError.