Reduce redundant database work in email pusher

Update the last stream ordering if the
`get_unread_push_actions_for_user_in_range_for_email` returns no new
push actions. This reduces the range that it needs to check next
iteration.

synapse/push/emailpusher.py

@@ -150,6 +150,10 @@ class EmailPusher(object):
 
         soonest_due_at = None
 
+        if not unprocessed:
+            yield self.save_last_stream_ordering_and_success(self.max_stream_ordering)
+            return
+
         for push_action in unprocessed:
             received_at = push_action['received_ts']
             if received_at is None: