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.

git-svn-id: https://develop.svn.wordpress.org/trunk@48210 602fd350-edb4-49c9-b593-d223f7449a82

src/js/_enqueues/wp/customize/controls.js

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