Additional fix to handle NULL lock_version

If the UI sends a string value for lock_version (e.g. "0"), then the previous monkey patch did not properly handle that properly. This commit casts the value to an integer to determine whether to look for NULL lock_versions.

For merge requests, we send a POST request to namespace/project/merge_requests/:iid with the merge_request[lock_version] parameter with a string 0. The string value comes from the form field, which explains why https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28145 wasn't sufficient.