Real-time collaboration: Remove ghost awareness state explicitly when refreshing.

Syncs PHP changes from WordPress/gutenberg#75883. Changes the real-time collaboration polling server endpoint to accept null for awareness values, which is used to explicitly disconnect users leaving the page and remove their awareness data more quickly.

CI run: #11049.

Fixes #64622.
Props alecgeatches.

git-svn-id: https://develop.svn.wordpress.org/trunk@61746 602fd350-edb4-49c9-b593-d223f7449a82

Author: ellatrix

src/wp-includes/collaboration/class-wp-http-polling-sync-server.php

@@ -122,7 +122,7 @@ public function register_routes(): void {
 			),
 			'awareness' => array(
 				'required' => true,
-				'type'     => 'object',
+				'type'     => array( 'object', 'null' ),
 			),
 			'client_id' => array(
 				'minimum'  => 1,