Fixed inconsistent check for isSubscriber method.#1757

ionutG-icoldo wants to merge 1 commit intomailchimp:develop-2.3mailchimp/mc-magento2:develop-2.3from

ionutG-icoldo:bugfix/is-subscriber-validationionutG-icoldo/mc-magento2:bugfix/is-subscriber-validationCopy head branch name to clipboard