Customize: Prevent selective refresh from causing infinite fallback refreshes...

Customize: Prevent selective refresh from causing infinite fallback refreshes when nav menu contains invalid items.

Invalid nav menu items are exported as settings in the customize controls since it `is_admin()`, but any such `nav_menu_item` settings are excluded from the preview since it's the frontend. Selective refresh for nav menus needs to ignore the syncing of any such `nav_menu_item` settings that are `_invalid` since they are never rendered, and when a nav menu only contains invalid items, an infinite fallback refresh can ensue.

Fixes #38890.

Built from https://develop.svn.wordpress.org/trunk@39333


git-svn-id: https://core.svn.wordpress.org/trunk@39273 1a063a9b-81f0-0310-95a4-ce76da25c4cd