add declare strict_type

Author: edenduong

app/code/Magento/SendFriend/Model/SendFriend.php

@@ -3,6 +3,8 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+declare(strict_types=1);
+
 namespace Magento\SendFriend\Model;
 
 use Magento\Framework\Exception\LocalizedException as CoreException;