Fix MR revert check when no merged_at is present
Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16678#note_56524899 - it was the spec that I wrote that didn't catch this case. The new specs do catch this.
Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16678#note_56524899 - it was the spec that I wrote that didn't catch this case. The new specs do catch this.