add IllegalArgumentException to DiscountType enum

Author: v-rudkovskiy

NetLicensingClient/src/main/java/com/labs64/netlicensing/domain/vo/DiscountType.java

@@ -13,6 +13,6 @@ public static DiscountType parseString(final String discountType) {
                 }
             }
         }
-        return null;
+        throw new IllegalArgumentException(discountType);
     }
 }