Issue #3514226: Add void return to functions in global namespace

Closes #3514226

core/includes/batch.inc

@@ -557,7 +557,7 @@ function _batch_finished() {
  * @see _batch_page()
  * @see drupal_register_shutdown_function()
  */
-function _batch_shutdown() {
+function _batch_shutdown(): void {
   if (($batch = batch_get()) && _batch_needs_update()) {
     \Drupal::service('batch.storage')->update($batch);
   }