Administration: Define the $title global on the Menus screen for classic themes.

This brings more consistency with other screens and avoids a PHP warning in `get_plugin_page_hookname()`:
{{{
preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated
}}}

Follow-up to [13257], [13366], [55263].

Props nendeb55, costdev, SergeyBiryukov.
Fixes #57918.

git-svn-id: https://develop.svn.wordpress.org/trunk@55552 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/nav-menus.php

@@ -28,6 +28,9 @@ if ( ! current_user_can( 'edit_theme_options' ) ) {
 	);
 }
 
+// Used in the HTML title tag.
+$title = __( 'Menus' );
+
 wp_enqueue_script( 'nav-menu' );
 
 if ( wp_is_mobile() ) {