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

@@ -1,7 +1,7 @@
 import os
 import sys
 from collections import defaultdict
-from contextlib import contextmanager
+from contextlib import contextmanager, suppress
 from functools import cached_property
 from typing import (
     TYPE_CHECKING,
@@ -374,16 +374,14 @@ def get_field_related_model_cls(self, field: Union["RelatedField[Any, Any]", For
                 # same model
                 related_model_cls = field.model
             elif "." not in related_model_cls:
-                # same file model
                 related_model_fullname = f"{field.model.__module__}.{related_model_cls}"
                 related_model_cls = self.get_model_class_by_fullname(related_model_fullname)
-                if related_model_cls is None:
-                    raise UnregisteredModelError
             else:
-                try:
+                with suppress(LookupError):
                     related_model_cls = self.apps_registry.get_model(related_model_cls)
-                except LookupError as e:
-                    raise UnregisteredModelError from e
+
+        if related_model_cls is None:
+            raise UnregisteredModelError
 
         return related_model_cls