fix(google/gemini): normalize falsy final_content to empty string to avoid None/empty responses

Author: owndev

pipelines/google/google_gemini.py

@@ -1758,7 +1758,7 @@ async def emit_chat_event(event_type: str, data: Dict[str, Any]) -> None:
 </details>""".strip()
                 final_content = f"{details_block}{final_answer_text}"
 
-            if final_content is None:
+            if not final_content:
                 final_content = ""
 
             # Ensure downstream consumers (UI, TTS) receive the complete response once streaming ends.