Avoid returning None from get_field_related_model_cls

#1495 updated
`get_field_related_model_cls` to raise `UnregisteredModelError` rather
than returning `None` for failure paths. However, None can still be
returned if the initial retrieval of `related_model_cls` returns None.

This patch adds a check to see if the initial retrieval has got a `None`
and then raises the appropriate error rather than letting `None` be
returned.

mypy_django_plugin/django/context.py

@@ -385,6 +385,9 @@ def get_field_related_model_cls(self, field: Union["RelatedField[Any, Any]", For
                 except LookupError as e:
                     raise UnregisteredModelError from e
 
+        if related_model_cls is None:
+            raise UnregisteredModelError
+
         return related_model_cls
 
     def _resolve_field_from_parts(