Replace isset() with empty() check. When the menu is not assigned to …

…the menu location, it returns 0 and generates a PHP warning. It will become a fatal error in PHP8. (#63)

includes/wp-api-menus-v2.php

@@ -267,7 +267,7 @@ public function get_menu_location( $request ) {
             $location   = $params['location'];
             $locations  = get_nav_menu_locations();
 
-            if ( ! isset( $locations[ $location ] ) ) {
+            if ( empty( $locations[ $location ] ) ) {
 	            return array();
             }