Fix UI error when the streaming response is empty

When the streaming response contains no stream, the UI shows something like:

```
{ "response": null, "status": "completed", "metadata": { "tokens_used": 992, "model_used": "anthropic" }, "requestId": "ai-request-1766165883584", "timestamp": "2025-12-19T17:38:06.908814", "workflow_created": true }
```

we should be able to catch that case and handle it properly