GH-102537: Handle check for PYTHONTZPATH failing in zoneinfo test (GH-102538)

GH-102537: Handle check for PYTHONTZPATH failing in zoneinfo test (GH-102538)

It is possible but unlikely for the `python_tzpath_context` function to fail between the start of the `try` block and the point where `os.environ.get` succeeds, in which case `old_env` will be undefined. In this case, we want to take no action.

Practically speaking this will really only happen in an error condition anyway, so it doesn't really matter, but we should probably do it right anyway.