Skip video summary production if yt-dlp fails silently

[rebeccacremona]

Background

We recently started capturing with --capture-video-as-attachment true in Perma.cc. We found that quite frequently, yt-dlp is in fact failing to extract a video or retrieve any metadata about a video, but is still returning 0 as though it had succeeded.

Examples:

• https://perma.cc/CY6E-4DTW?embed=False
• https://perma.cc/2LVU-7P27?embed=False

(Use the RWP side menu to view Pages, and navigate to "Extracted video data")

This PR

This PR makes the tiniest of tweaks:

• It checks to see if there's any metadata in the metadata file, so that we don't create an empty "generated exchange" when there isn't.
• It checks to see if any generated exchanges were added to the archive, either videos, subtitles, or video metadata files, before generating the video summary page, so that "empty" summaries aren't added to the WACZ.

I may not have added these tweaks idiomatically; happy to adjust in any way.

[rebeccacremona]

I made a few suggestions that you should feel free to ignore. This is great, thanks @rebeccacremona

Thank you @matteocargnelutti! This is exactly what I meant by, "I may not have added these tweaks idiomatically"