GitLab

fixes regression from #7471

test plan:
1. start a conversation with another one or more users
2. delete one of the users
3. ensure the conversation still loads (you should see all participants
   and messages)

Change-Id: Ic0e567f22010864054f7396553b43173110be317
Reviewed-on: https://gerrit.instructure.com/9211

Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>