Static fallback to 19% in basket

[mitelg]

Instructions for reproduction:

• Basic settings > Create a shopping cart discount for customer groups, e.g. 10%.
• Create a tax rule for this customer group and define 0%-> Shopping cart discount is still calculated with 19%.

The cause is this query: https://github.com/shopware5/shopware/blob/v5.7.18/engine/Shopware/Core/sBasket.php#L425-L427

If you change this e.g. to (!$tax && $tax != 0) then it also works with 0% tax.