Avoid notice on checkout confirmation

Ensures that the shipping tax group exists before adding to it.
This avoids a notice that otherwise appears on the checkout
confirmation page.

includes/modules/order_total/ot_shipping.php

@@ -53,6 +53,9 @@ function process() {
           $shipping_tax_description = tep_get_tax_description($GLOBALS[$module]->tax_class, $order->delivery['country']['id'], $order->delivery['zone_id']);
 
           $order->info['tax'] += tep_calculate_tax($order->info['shipping_cost'], $shipping_tax);
+          if (!isset($order->info['tax_groups']["$shipping_tax_description"])) {
+            Guarantor::guarantee_subarray($order->info, 'tax_groups')["$shipping_tax_description"] = 0;
+          }
           $order->info['tax_groups']["$shipping_tax_description"] += tep_calculate_tax($order->info['shipping_cost'], $shipping_tax);
           $order->info['total'] += tep_calculate_tax($order->info['shipping_cost'], $shipping_tax);