Guard against None context ID in append_to_audio_context

After interruption, both _playing_context_id and _turn_context_id are
None. If a subclass calls append_to_audio_context(None, frame), the
recovery path matches (None == None) and creates a bogus audio context
that blocks the handler from ever processing the real context.

Early-return when context_id is falsy to prevent this.

src/pipecat/services/tts_service.py

@@ -1182,6 +1182,9 @@ class TTSService(AIService):
             context_id: The context to append audio to.
             frame: The audio or control frame to append.
         """
+        if not context_id:
+            logger.debug(f"{self} unable to append audio to context: no context ID provided")
+            return
         if self.audio_context_available(context_id):
             logger.trace(f"{self} appending audio {frame} to audio context {context_id}")
             await self._audio_contexts[context_id].put(frame)