fix: only consider vat === null, 0 is allowed

src/QUI/ERP/Accounting/Calc.php

@@ -235,7 +235,7 @@ public function calcArticleList(ArticleList $List, $callback = false)
                 $priceFactorValue = $PriceFactor->getValue();
                 $vatValue         = $PriceFactor->getVat();
 
-                if ($vatValue === 0) {
+                if ($vatValue === null) {
                     $vatValue = QUI\ERP\Tax\Utils::getTaxByUser($this->getUser())->getValue();
                 }