fix: add default value for pcm timestamp

Author: YiminW

interface/ten_ai_base/tts2.py

@@ -160,7 +160,7 @@ async def _process_input_queue(self, ten_env: AsyncTenEnv) -> None:
 
 
     async def send_tts_audio_data(
-        self, audio_data: bytes, timestamp: int
+        self, audio_data: bytes, timestamp: int = 0
     ) -> None:
         """End sending audio out."""
         try: