code: public static Money add(Money money1, Money money2) {
          if (money1 == null) {
              return money2;
          }
          if (money2 == null) {
              return money1;
          }
          return money1.plus(money2);
      }
docs: Adds two {@code Money} objects, handling null.
  <p>
  This returns {@code money1 + money2} where null is ignored.
  If both input values are null, then null is returned.
  
  @param money1  the first money instance, null returns money2
  @param money2  the first money instance, null returns money1
  @return the total, where null is ignored, null if both inputs are null
  @throws CurrencyMismatchException if the currencies differ