Customize: Ensure a `custom_css` post insertion gets an initial post revision.

Props georgestephanis, westonruter.
See #30854, #38672, #35395.
Fixes #39032.

Built from https://develop.svn.wordpress.org/trunk@39477


git-svn-id: https://core.svn.wordpress.org/trunk@39417 1a063a9b-81f0-0310-95a4-ce76da25c4cd