fix: handle empty menu

if menu exists but is empty we need an empty array rather than null

src/Models/MenuItem.php

@@ -115,7 +115,7 @@ private static function getIndexedWPNavMenuItems(string $type): ?array
         }
 
         // If the menu exists, get its items.
-        $items = wp_get_nav_menu_items($menu->term_id, ['update_post_term_cache' => false]) ?: null;
+        $items = wp_get_nav_menu_items($menu->term_id, ['update_post_term_cache' => false]) ?: [];
 
         $indexedItems = [];
         foreach ($items as $item) {