[PIWOO-348] FIX: Apple Pay button payment not possible as a guest user

[inpsyde-maticluznar]

When debugging was enabled on servers with PHP8+, some server responses included warnings which resulted in broken frontend functionality. This simple fix takes care of that:

if (!isset($gatewaySettings['payment_surcharge'])) {
       return 0.0;
}

Other changes are from the code formatting.