Test non-nullable relationships

I can't test this now, because you can't do an initial sync with non-nullable relationships. I would have to run an initial sync, then follow up with another sync that adds in the non-nullable field.

Once #43 and maybe #143 are done, I can do this.

Review issues #43 and #143 first, since the requested test depends on their work. Once an initial sync supports non-nullable relationships, add coverage for that relationship case and verify that the follow-up sync is no longer required.

Written by the indexing model from the issue text.