MAGETWO-66328: Remove usages of serialize/unserialize in CustomerSampleData

- Fix docblock

Author: tjwiebell

app/code/Magento/CustomerSampleData/Model/Customer.php

@@ -89,7 +89,7 @@ class Customer
      * @param \Magento\Store\Model\StoreManagerInterface $storeManager
      * @param \Magento\Framework\Api\DataObjectHelper $dataObjectHelper
      * @param \Magento\Framework\App\State $appState
-     * @param Json $serializer
+     * @param Json|null $serializer
      * @SuppressWarnings(PHPMD.ExcessiveParameterList)
      */
     public function __construct(