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