Video storyboard with empty frames with provisioned content

Reproduction steps:
1. Configure a server v2021.1
2. Install packages nuxeo-showcase-content and nuxeo-web-ui
3. Launch the server
4. Note that Coastline.mp4 and Keyboard.mp4 are missing thumbnails but River.mp4 is ok.

This happens because the vid:storyboard property of the document contains an array filled with invalid frames (i.e., [\{comment: null, content: null, timecode: null},\{comment: null, content: null, timecode: null}, ...]). However, if we re-upload the file, the frames are correctly returned and displayed.

Note that this is also reproducible on Nightly: https://nightly.nuxeo.com/nuxeo/ui/#!/browse/default-domain/workspaces/Sample%20Content/Videos/Coastline.mp4, and is also reproducible on the NCO test environments: https://nxdev-webui-11.beta.nuxeocloud.com/nuxeo/ui/#!/browse/default-domain/workspaces/Sample%20Content/Videos/Coastline.mp4

This causes one test on RF to fail.