Don't create empty images in /rows
Don't create empty images. Otherwise on a second call to /rows it will return the first empty image.
We were already doing something similar for audio.

Fix https://github.com/huggingface/datasets-server/issues/2297

TODO after merged:
- [x] delete the cached assets for HuggingFaceM4/WebSight to remove the empty images
Don't create empty images in /rows: Don't create empty images. Otherwise on a second call to /rows it will return the first empty image.
We were already doing something similar for audio.

Fix https://github.com/huggingface/datasets-server/issues/2297

TODO after merged:
- [x] delete the cached assets for HuggingFaceM4/WebSight to remove the empty images