Scripts: Fix a PHP error in admin-ajax calls.

When making an ajax request, `wp_enqueue_registered_block_scripts_and_styles()` checked if `is_admin()` was true before accessing `$current_screen`, rather than checking if `$current_screen` was defined. This is usually fine, execept for in ajax requests.

Props ocean90, foreverpinetree, pento.
Merges [43893] and [43894] to trunk.
Fixes #45302.
Built from https://develop.svn.wordpress.org/trunk@44249


git-svn-id: https://core.svn.wordpress.org/trunk@44079 1a063a9b-81f0-0310-95a4-ce76da25c4cd