Final product test changes

Author: fracz

src/main/java/pl/edu/agh/mwo/invoice/Invoice.java

@@ -17,7 +17,7 @@ public void addProduct(Product product, Integer quantity) {
     }
 
     public BigDecimal getSubtotal() {
-        return null;
+        return BigDecimal.ZERO;
     }
 
     public BigDecimal getTax() {

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

@@ -13,6 +13,9 @@ protected Product(String name, BigDecimal price, BigDecimal tax) {
         if (name == null || name.equals("")) {
             throw new IllegalArgumentException("Product name cannot be null");
         }
+        if (price == null || price.compareTo(BigDecimal.ZERO) == -1) {
+            throw new IllegalArgumentException("Price is invalid");
+        }
         this.name = name;
         this.price = price;
         this.taxPercent = tax;