Register RTC meta keys for all post types

Use empty string subtype so the keys are found during cache
invalidation lookups, which do not have access to the post type.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: ellatrix

src/wp-includes/post.php

@@ -692,15 +692,15 @@ function create_initial_post_types() {
 		);
 
 		register_post_meta(
-			'wp_sync_storage',
+			'',
 			'wp_sync_awareness',
 			array(
 				'invalidates_query_cache' => false,
 			)
 		);
 
 		register_post_meta(
-			'wp_sync_storage',
+			'',
 			'wp_sync_update',
 			array(
 				'invalidates_query_cache' => false,