ENGCOM-3006: Fix shipping discount failed to apply during place order #18098

 - Merge Pull Request #18098 from torreytsui/magento2:feature/fix-shipping-discount-failed-to-apply
 - Merged commits:
   1. 1010526

Author: magento-engcom-team

app/code/Magento/Tax/Model/Sales/Total/Quote/Tax.php

@@ -164,9 +164,8 @@ protected function clearValues(Address\Total $total)
         $total->setBaseShippingInclTax(0);
         $total->setShippingTaxAmount(0);
         $total->setBaseShippingTaxAmount(0);
-        $total->setShippingAmountForDiscount(0);
-        $total->setBaseShippingAmountForDiscount(0);
-        $total->setBaseShippingAmountForDiscount(0);
+        $total->setShippingAmountForDiscount(null);
+        $total->setBaseShippingAmountForDiscount(null);
         $total->setTotalAmount('extra_tax', 0);
         $total->setBaseTotalAmount('extra_tax', 0);
     }