Full ProductTest working

Author: fracz

src/main/java/pl/edu/agh/mwo/invoice/product/Product.java

@@ -10,6 +10,12 @@ public abstract class Product {
     private final BigDecimal taxPercent;
 
     protected Product(String name, BigDecimal price, BigDecimal tax) {
+        if (name == null || name.isEmpty()) {
+            throw new IllegalArgumentException("Product name cannot be null or empty.");
+        }
+        if (price == null || price.signum() == -1) {
+            throw new IllegalArgumentException("Price name cannot be null.");
+        }
         this.name = name;
         this.price = price;
         this.taxPercent = tax;