Fix: adds a defense to avoid attempting an unserialize action on an array

[loneken79]

This case happens when dumping config from magento 2 with an empty braintree config Then $countryCardTypes is save as an empty array in app/etc/config.php (and not a string of an empty array) Magento 2 Import config does not fails But when the framework try to read the config (when users access the cart page) then an unserialize is attempted which leads to an Exception