fix yield error

app.py

@@ -8,7 +8,8 @@ async def process_audio_stream(audio_path, max_tokens):
     Process audio with streaming response via WebSocket
     """
     if not audio_path:
-        return "Please upload or record an audio file first."
+        yield "Please upload or record an audio file first."
+        return
     
     try:
         # Read audio file