Meta Boxes: Sync checks for valid meta boxes from `do_meta_boxes()` to...

Meta Boxes: Sync checks for valid meta boxes from `do_meta_boxes()` to `the_block_editor_meta_boxes()`.

Merges [43792] from the 5.0 branch to trunk.

Props ocean90.
See #45112.

Built from https://develop.svn.wordpress.org/trunk@44141


git-svn-id: https://core.svn.wordpress.org/trunk@43971 1a063a9b-81f0-0310-95a4-ce76da25c4cd