typing: Add more tests for TypeVar (#104571)

typing: Add more tests for TypeVar (#104571)

During the PEP 695 implementation at one point I made
TypeVar.__name__ return garbage, and all of test_typing passed.
So I decided to add a few more tests. In the process I discovered
a minor incompatibility from the C implementation of TypeVar:
empty constraints were returned as None instead of an empty tuple.