Issue 43455: pathlib mistakenly assumes os.getcwd() is a resolved path in Windows

pathlib._WindowsFlavour.resolve() mistakenly assume that os.getcwd() returns a resolved path in Windows:

    s = str(path)
    if not s:
        return os.getcwd()

I don't think this is a practical problem since `str(path)` should never be an empty string. But if there is a concern that the result is an empty string, the code should use `s = str(path) or '.'`, and resolve "." like any other relative path.

In POSIX the result of getcwd() "shall contain no components that are dot or dot-dot, or are symbolic links". In Windows, os.getcwd() calls WinAPI GetCurrentDirectoryW(), which returns a fully-qualified path that may contain symbolic components that would be resolved in a final path. This includes filesystem symlinks and bind mounts (junctions), as well as mapped and substitute drives (i.e. drives that resolve to a filesystem directory instead of a volume device).