Customizer: Fix saving menus with empty names or names that are already used.

Adds validation for initially-supplied nav menu name, blocking empty names from being supplied. If later an empty name is supplied and the nav menu is saved, the name "(unnamed)" will be supplied instead and supplied back to the client. If a name is supplied for the menu which is currently used by another menu, then the name conflict is resolved by adding a numerical counter similar to how `post_name` conflicts are resolved. Includes unit tests.

Fixes #32760.


Built from https://develop.svn.wordpress.org/trunk@33071


git-svn-id: http://core.svn.wordpress.org/trunk@33042 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-nav-menus.php

@@ -281,6 +281,7 @@ final class WP_Customize_Nav_Menus {
 			'itemTypes'            => $this->available_item_types(),
 			'l10n'                 => array(
 				'untitled'          => _x( '(no label)', 'missing menu item navigation label' ),
+				'unnamed'           => _x( '(unnamed)', 'Missing menu name.' ),
 				'custom_label'      => __( 'Custom Link' ),
 				/* translators: %s: Current menu location */
 				'menuLocation'      => __( '(Currently set to: %s)' ),