Fix regression introduced by 5af27f30adababcb837b647a68f1a38249d55b2f

The problem was that the method used to return `nil` in case the lock was not present, but after that commit it started returning a "\n"

I didn't update the changelog as it is a bug that was not present on a previous version of the gem ;-)

/cc @dirs @TiuTalk @guiloyins @marcioj