Media: Fix regression with display of small images in media library.

Fixes a regression in the media library after [38949], which caused
small images (< thumbnail size) to not display within the media library.

Accounts for images that have no intermediate sizes
in `wp_prepare_attachment_for_js()`.

Adds test.

Fixes #38965.
Props joemcgill, clorith, mikeschroder.
Built from https://develop.svn.wordpress.org/trunk@39396


git-svn-id: https://core.svn.wordpress.org/trunk@39336 1a063a9b-81f0-0310-95a4-ce76da25c4cd