services(gemini_multimodal_live): set content to [] if not present in messages

... which it will be if the message is a tool call

src/pipecat/services/gemini_multimodal_live/audio_transcriber.py

@@ -70,7 +70,7 @@ class AudioTranscriber:
 
             history = ""
             for msg in previous_messages:
-                content = msg.get("content")
+                content = msg.get("content", [])
                 if isinstance(content, str):
                     history += f"{msg.get('role')}: {content}\n"
                 else: