Check that menu items are objects in _wp_menu_item_classes_by_context() - #13721

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Tested and Verified ✅

Environment:

• WordPress: 7.2-alpha (trunk)
• PHP: 8.3

Reproduction & Verification:

1. Before patch (trunk):
   Hooked into wp_get_nav_menu_items and returned an array with a null item:

   add_filter( 'wp_get_nav_menu_items', function ( $items ) {
       if ( ! empty( $items ) ) {
           $items[0] = null;
       }
       return $items;
   } );

Calling wp_nav_menu() fails with fatal error:
Fatal error: Uncaught Error: Attempt to assign property "current" on null in src/wp-includes/nav-menu-template.php:399

2. After applying PR Check that menu items are objects in _wp_menu_item_classes_by_context() #13721:
   The fatal error is eliminated; non-object entries are safely stripped from $menu_items.
   The navigation menu renders as expected with the remaining valid items intact.
   Ran vendor/bin/phpunit --filter test_wp_nav_menu_should_skip_non_object_menu_items: Pass (OK (1 test, 2 assertions)).
   Ran full menu test suite vendor/bin/phpunit --group menu: Pass (OK (106 tests, 238 assertions)).
   PHPCS check: 0 errors, 0 warnings.