add declare strict_type

Author: edenduong

app/code/Magento/Customer/Controller/Account/CreatePost.php

@@ -3,6 +3,8 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
+
 namespace Magento\Customer\Controller\Account;
 
 use Magento\Customer\Api\CustomerRepositoryInterface as CustomerRepository;