Fix editing issues and MRs with NULL lock_version

What does this MR do?

When we upgraded to Rails 5.1.7, we thought we could remove the monkey patch for handling NULL lock_versions since this bug was supposedly fixed in https://github.com/rails/rails/pull/26050. However, that pull request does NOT fix the problem. The issue is that the value read in https://github.com/rails/rails/blob/8efdd545f3d40c6fa9ea615a196264f813c0c6b4/activerecord/lib/active_record/locking/optimistic.rb#L83 returns 0 rather than nil. This is used to signify what version the user got before making the change, rather than the raw value stored in the database.

This merge request also adds a test that shows https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24050 worked. We removed this in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27480.

We can't re-add the patch as-is because the underlying code changed in Rails 5.1: https://github.com/rails/rails/blob/5-1-stable/activerecord/lib/active_record/locking/optimistic.rb