AssertionError when GitLab calls a push hook for a repository without OAuthCredentials

Sentry Issue: ARKINDEX-BACKEND-5P

AssertionError: No credentials available
(4 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 505, in dispatch
    response = self.handle_exception(exc)
  File "rest_framework/views.py", line 465, in handle_exception
    self.raise_uncaught_exception(exc)
  File "rest_framework/views.py", line 476, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 502, in dispatch
    response = handler(request, *args, **kwargs)
  File "arkindex/dataimport/api.py", line 513, in post

The hook could maybe return a 403 or a 404 instead, using a properly filtered queryset.