GH-79634: Speed up pathlib globbing by removing `joinpath()` call. (#114623)

Remove `self.joinpath('')` call that should have been removed in 6313cdde.

This makes `PathBase.glob('')` yield itself *without* adding a trailing slash. It's hard to say whether this is more or less correct, but at least everything else is faster, and there's no behaviour change in the public classes where empty glob patterns are disallowed.