REST API: Fix bug where comment author and author email could be an empty...

REST API: Fix bug where comment author and author email could be an empty string when creating a comment.

If the `require_name_email` option is true, creating a comment with an empty string for the author name or email should not be accepted.  Both values can be an empty string on update.

Props flixos90, hnle, dd32, rachelbaker, jnylen0, ChopinBach, joehoyle, pento.

Fixes #38971.

Built from https://develop.svn.wordpress.org/trunk@39444


git-svn-id: https://core.svn.wordpress.org/trunk@39384 1a063a9b-81f0-0310-95a4-ce76da25c4cd