[Forwardport] Add statement to 'beforeSave' method to allow app:config:import

[adrian-martinez-interactiv4]

Original Pull Request

magento#14829

Fix to app:config:import for beforeSave method on Braintree CountryCreditCard class.

Description

magento@04065890c38d:/srv/magento2$ php bin/magento app:config:import
Processing configurations data from configuration file...
Import failed: Warning: Invalid argument supplied for foreach() in /srv/magento2/vendor/magento/module-braintree/Model/Adminhtml/System/Config/CountryCreditCard.php on line 70

Manual testing scenarios

1. php bin/magento app:config:dump
2. change some configs
3. php bin/magento app:config:import

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)