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.
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