docs: mark WP_Dependencies::$all_queued_deps` as nullable.

Author: justlevine

src/wp-includes/class-wp-dependencies.php

@@ -91,7 +91,7 @@ class WP_Dependencies {
 	 *
 	 * @since 5.4.0
 	 *
-	 * @var array
+	 * @var ?array
 	 */
 	private $all_queued_deps;