youtube: duration field is null in flat-playlist --json output

[tonyc-blastpoint]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2021.01.24.1
• I've searched the bugtracker for similar site feature requests including closed ones

Description

WRITE DESCRIPTION HERE

The duration field is empty when fetching a YouTube playlist when using the flat playlist JSON output:

$  python -m youtube_dl --flat-playlist -j https://www.youtube.com/user/BostonDynamics/videos

{"_type": "url_transparent", "ie_key": "Youtube", "id": "F843AqfNsaA", "url": "F843AqfNsaA", "title": "Meet Spot Enterprise", "description": null, "duration": null, "view_count": 112689, "uploader": null}
{"_type": "url_transparent", "ie_key": "Youtube", "id": "6Zbhvaac68Y", "url": "6Zbhvaac68Y", "title": "Spot's Got an Arm!", "description": null, "duration": null, "view_count": 1333404, "uploader": null}
{"_type": "url_transparent", "ie_key": "Youtube", "id": "fn3KWM1kuAw", "url": "fn3KWM1kuAw", "title": "Do You Love Me?", "description": null, "duration": null, "view_count": 27911156, "uploader": null}

#25853 was submitted to fix this functionality (among other things) but not merged. A fix was requested in #25720 but closed.

[Crypto90]

I already made some quick and dirty changes to get the playlist results right:
https://github.com/Crypto90/youtube-dl-1

I compile it always myself because the patches never got merged...
those changes work with the latest code. maybe someone can look over it/improve it/beautify it to get it merged.