Add updated history count to dao_update_notifications_by_reference

For returned letter updates most notifications won't exist in the
notifications table, so in order to find out whether the reference
matches any known letters we need to check the count of updated
history records.

app/celery/letters_pdf_tasks.py

@@ -288,7 +288,7 @@ def update_letter_pdf_status(reference, status):
         update_dict={
             'status': status,
             'updated_at': datetime.utcnow()
-        })
+        })[0]
 
 
 def replay_letters_in_error(filename=None):