EmbedGallery_ViewImage.Thumbnail is a non-pointer string (unlike
EmbedVideo_View.Thumbnail), so an empty value would slip through as
<meta property="og:image" content="">. Belt-and-suspenders guard
in galleryThumbs, with the defensive test extended to cover the case.
Also adds a parallel hidden-embed test for gallery alongside the
existing images variant. Functionally redundant with the early-return
at the top of extractPostMedia, but asserts the contract directly
against the gallery branch so it survives any future refactor that
moves the hide check into a per-shape branch.
Michael Black
2c04d28a9b