MAG2-184: Fix for empty creditcard-type in Info-block

[berwa]

Reason for the change During the changes in https://github.com/PAYONE-GmbH/magento-2/commit/342404b808c772145ab3465daa3131ac2144cab7#diff-57bbdd480bb30e81d51d4183f83e3b47c0347f0658a1793e054f3739c44ef065 the structure of the array was changed, but the implementation of getCreditcardType does not represent those changes up to now. As the Payone-Server still returns the 1-letter code of the creditcard-type (e.g. 'V' for Visa) the current implementation finds no entry in the array and returns an empty string. This results in an empty creditcard-type entry in the order-details of the Magento backend and order confirmation e-mails.

Steps to verify the change

• Place an order with payment-method creditcard, wait for appointed-call to arrive
• Check order details in Magento backend and order-confirmation e-mail for filled creditcard-type

[fjbender]

Thank you for your contribution! :)