✅ add test for empty invoice total tax field

Author: ianardee

src/test/java/com/mindee/product/invoice/InvoiceV4Test.java

@@ -45,6 +45,7 @@ void whenEmptyDeserialized_mustHaveValidProperties() throws IOException {
     Assertions.assertTrue(docPrediction.getTaxes().isEmpty());
     Assertions.assertTrue(docPrediction.getSupplierPaymentDetails().isEmpty());
     Assertions.assertNull(docPrediction.getSupplierName().getValue());
+    Assertions.assertNull(docPrediction.getTotalTax().getValue());
     Assertions.assertTrue(docPrediction.getSupplierCompanyRegistrations().isEmpty());
     Assertions.assertNull(docPrediction.getSupplierAddress().getValue());
     Assertions.assertNull(docPrediction.getCustomerName().getValue());