Fixed #24418 -- Prevented crash in refresh_from_db with null fk

Thanks Johannes Lerch for the report, Tim Graham for the test case,
and Simon Charette for the review.

django/db/models/base.py

@@ -596,7 +596,7 @@ class Model(six.with_metaclass(ModelBase)):
             if field.rel and field.get_cache_name() in self.__dict__:
                 rel_instance = getattr(self, field.get_cache_name())
                 local_val = getattr(db_instance, field.attname)
-                related_val = getattr(rel_instance, field.related_field.attname)
+                related_val = None if rel_instance is None else getattr(rel_instance, field.related_field.attname)
                 if local_val != related_val:
                     del self.__dict__[field.get_cache_name()]
         self._state.db = db_instance._state.db

tests/basic/tests.py

@@ -735,6 +735,13 @@ class ModelRefreshTests(TestCase):
             self.assertFalse(hasattr(s3_copy.selfref, 'touched'))
             self.assertEqual(s3_copy.selfref, s2)
 
+    def test_refresh_null_fk(self):
+        s1 = SelfRef.objects.create()
+        s2 = SelfRef.objects.create(selfref=s1)
+        s2.selfref = None
+        s2.refresh_from_db()
+        self.assertEqual(s2.selfref, s1)
+
     def test_refresh_unsaved(self):
         pub_date = self._truncate_ms(datetime.now())
         a = Article.objects.create(pub_date=pub_date)