[Pixiv] Metadata caption field empty on some works that have captions

[Meta-Didact]

Preface:
Pixiv work metadata includes a field caption that is the description that the artist has given the work.

Issue:
Occasionally a work that has a caption when viewing in a browser, but when downloaded with gallery-dl the field in the metadata output is an empty string ''.

The issue seems intermittent. Sometimes on redownloading the field is correctly populated.

Remedy:
As the issue comes and goes it likely is an issue on Pixiv's side and thus cannot be solved by gallery-dl but I felt it worthwhile documenting it here so that if someone is affected they may want to try download the affected works again.

[mikf]

This is a known issue (#4327 (comment)).

Only solution would be not using the App API, like PixivUtil2
or some form of fallback (#4327 (comment)).