Customize: Ensure the default message in `LockedNotification` is set to a string instead of `null`.

This avoids a JS error when passing the message to `wp.a11y.speak()` when showing the "changeset is locked" overlay to prevent two users from editing the same changeset simultaneously.

Props dlh.
Fixes #50500.
Built from https://develop.svn.wordpress.org/trunk@48210


git-svn-id: http://core.svn.wordpress.org/trunk@47979 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -8128,6 +8128,7 @@
 					_code = code || 'changeset_locked';
 					_params = _.extend(
 						{
+							message: '',
 							type: 'warning',
 							containerClasses: '',
 							lockUser: {}

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-alpha-48209';
+$wp_version = '5.5-alpha-48210';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.