LCORE-464: fix potential None reference handling

Author: tisnik

src/app/endpoints/query.py

@@ -492,8 +492,11 @@ async def retrieve_response(  # pylint: disable=too-many-locals
             # Metric for LLM validation errors
             metrics.llm_calls_validation_errors_total.inc()
             break
+    if response is not None and response.output_message is not None:  # type: ignore[union-attr]
+        return str(response.output_message.content), conversation_id  # type: ignore[union-attr]
 
-    return str(response.output_message.content), conversation_id  # type: ignore[union-attr]
+    logger.warning("Response does not contain any message to be returned")
+    return "", conversation_id  # fallback
 
 
 def validate_attachments_metadata(attachments: list[Attachment]) -> None: