Treat empty/non-str library overrides as unset

[Pitchfork-and-Torch]

Summary

Only treat non-empty str values from thread_local.name / current_async_library_cvar as library overrides.

Previously any non-None value was returned, so "" or an int leaked as a false library name. Invalid overrides are ignored and sniffing continues.

Test plan

• sniffio/_tests/test_sniffio.py (incl. test_empty_name_ignored) passes

[x42005e1f]

For type checking, there are already type hints. In this regard, runtime checks are redundant.

As for empty strings, that makes even less sense. Your PR treats them as equivalent to None, and that violates PEP 20 – The Zen of Python ("There should be one-- and preferably only one --obvious way to do it"; though it is hard to imagine a situation where someone would want to explicitly set None instead of using a set-reset pattern). And even if you took a different approach and raised an exception for "", it is just one of countless invalid values (arbitrary sequences of spaces, typos, etc.), and they would be detected at runtime anyway.

[A5rocks]

I think setting this to an empty string is just like, sure whatever? Why should we assume that library code that integrates with sniffio, which is probably like 2 lines in a heavily tested async framework, can mess up this badly?