fix: ensure valid chunk text before yielding response in ChatService

Author: Pavan-Microsoft

src/api/services/chat_service.py

@@ -157,8 +157,9 @@ async def stream_openai_text(self, conversation_id: str, query: str) -> Streamin
                                     citations.extend(content.annotations)
 
                         if first_chunk:
-                            yield "{ \"answer\": " + str(chunk.text)
-                            first_chunk = False
+                            if chunk is not None and chunk.text != "":
+                                first_chunk = False
+                                yield "{ \"answer\": " + str(chunk.text)
                         else:
                             complete_response += str(chunk.text)
                             yield str(chunk.text)