stat-util: ignore hidden_or_backup_file when checking if dir is empty

stat-util: ignore hidden_or_backup_file when checking if dir is empty

Commit https://github.com/systemd/systemd/commit/a068aceafbf
changed dir_is_emtpy_at to use FOREACH_DIRENT_IN_BUFFER instead of
FOREACH_DIRENT, but used dot_or_dotdot which just checks if the name
is literally '.' or '..' which is not enough, previous behaviour was
to ignore all hidden files, so restore that and add a test case.

Fixes https://github.com/systemd/systemd/issues/23220