[YouTube] Fallback to playlist uploader on course playlists

[FineFindus]

Course playlists videos do not contain the uploader. This leads to an exception being thrown when trying to extract the uploader values. We can fallback to the playlist uploader values, as every video is uploaded by the playlist uploader.

Closes: #1425

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[TobiGr]

I think the fallback is only correct for course playlists, isn't it? Is the PLAYLIST_VIDEO_RENDERER used for other plylists types as well?
If this is the case, we should check if a fallback uploader is correct for them, too. If PLAYLIST_VIDEO_RENDERER is only used for course playlists, we should document it somewhere.

[FineFindus]

I think the fallback is only correct for course playlists, isn't it? Is the PLAYLIST_VIDEO_RENDERER used for other playlists types as well?

Depends on what videos the playlist contains, but yes, they do use, PLAYLIST_VIDEO_RENDERER as well.

If this is the case, we should check if a fallback uploader is correct for them, too

It likely isn't (unless some create created it for their own videos), however I don't think there are currently any cases, where there is a playlist that doesn't provide the uploader for the individual items, so the fallback wouldn't be triggered. Happy to use the fallback only for courses either way though.