docs: add missing null to _wp_get_current_user() return type

Co-authored-by: Mukesh Panchal <mukeshpanchal27@users.noreply.github.com>

Author: justlevine

src/wp-includes/user.php

@@ -3708,7 +3708,7 @@ function wp_get_users_with_no_role( $site_id = null ) {
  * @see wp_get_current_user()
  * @global WP_User $current_user Checks if the current user is set.
  *
- * @return WP_User Current WP_User instance.
+ * @return WP_User|null Current WP_User instance if user is logged in, null otherwise.
  */
 function _wp_get_current_user() {
 	global $current_user;