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

@@ -369,6 +369,9 @@ def get_field_related_model_cls(self, field: Union["RelatedField[Any, Any]", For
         else:
             related_model_cls = field.field.model
 
+        if related_model_cls is None:
+            raise UnregisteredModelError
+
         if isinstance(related_model_cls, str):
             if related_model_cls == "self":  # type: ignore[unreachable]
                 # same model