Guard uses of the keep alive thread in case they're None

Fixes #5800

discord/gateway.py

@@ -398,10 +398,12 @@ class DiscordWebSocket:
                 raise ReconnectWebSocket(self.shard_id)
 
             if op == self.HEARTBEAT_ACK:
+                if self._keep_alive:
                     self._keep_alive.ack()
                 return
 
             if op == self.HEARTBEAT:
+                if self._keep_alive:
                     beat = self._keep_alive.get_payload()
                     await self.send_as_json(beat)
                 return