Don't flush previous backups when no playlist were found
In case we're trying to generate a backup and no playlist is found, we probably shouldn't purge a previous backup, as we might be in a situation where no playlist was restored, but we still have a valid backup, which we don't want to delete
Not sure this is an actual problem, but vlc-android#1119 (closed) makes me think we should be conservative when flushing backups.