[1.9.x] Fixed #25715 -- Fixed Model.refresh_from_db() with ForeignKey w/on_delete=SET_NULL.

Backport of 54e2e688e1 from master

tests/basic/models.py

@@ -34,6 +34,7 @@ class SelfRef(models.Model):
         null=True, blank=True,
         related_name='+',
     )
+    article = models.ForeignKey(Article, models.SET_NULL, null=True, blank=True)
 
     def __str__(self):
         # This method intentionally doesn't work for all cases - part