If SRS responds with this empty data packet, FFmpeg will receive an
empty stream, like `Stream #0:0: Data: none` in following logs:
```bash
ffmpeg -i rtmp://localhost:11935/live/livestream
```
This won't cause the player to fail, but it will inconvenience the user
significantly. It may also cause FFmpeg slower to analysis the stream,
see #3767
---------
Co-authored-by: Jacob Su <suzp1984@gmail.com>
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