fix(superuser_models_pre_oci): have None for approver

### Description of Changes

this fixes a null pointer exception

Issue: https://coreosdev.atlassian.net/browse/QUAY-fix_bug_superuser_panel

## Reviewer Checklist

- [ ] It works!
- [ ] Comments provide sufficient explanations for the next contributor
- [ ] Tests cover changes and corner cases
- [ ] Follows Quay syntax patterns and format

endpoints/api/superuser_models_pre_oci.py

@@ -38,6 +38,8 @@ def _create_log(log, log_kind):
 
 
 def _create_user(user):
+  if user is None:
+    return None
   return User(user.username, user.email, user.verified, user.enabled, user.robot)