The background images for image and video blocks show at publishing empty blocks of these

To reproduce:

1. Open SJRK staging site;
2. Click add block buttons to add 4 empty blocks. Each of them is for text, image, audio, video;
3. Click "Continue" button. On the next page, click "preview my story" button;
4. The background images for image and video blocks show. The background image for the audio block doesn't show.

The expected behavior is none of these background images should show when adding empty blocks.