transports(daily): make sure we don't send data if client is closed

src/pipecat/transports/services/daily.py

@@ -199,6 +199,9 @@ def set_callbacks(self, callbacks: DailyCallbacks):
         self._callbacks = callbacks
 
     async def send_message(self, frame: DailyTransportMessageFrame):
+        if not self._client:
+            return
+
         future = self._loop.create_future()
         self._client.send_app_message(
             frame.message,