transports(daily): don't send messages if not joined

src/pipecat/transports/services/daily.py

@@ -241,7 +241,7 @@ def set_callbacks(self, callbacks: DailyCallbacks):
         self._callbacks = callbacks
 
     async def send_message(self, frame: TransportMessageFrame | TransportMessageUrgentFrame):
-        if not self._client:
+        if not self._joined or self._leaving:
             return
 
         participant_id = None