Differentiate empty repository restores

Closes gitlab#357044 (closed)

This MR detects when the repository backup is for an empty repository. Creating empty repository backups was completed in !6090 (merged)

The refs file in backups were introduced with incremental backups in %15.3 via gitlab#355945 (closed). Since it is forbidden to restore a backup from a different major release, we can expect that all backups have refs files. See https://gitlab.com/gitlab-org/gitlab/-/blob/de261c5648ce232c4d1a62c77cab98c6601d4c24/lib/backup/manager.rb#L420