I have a build() method that creates a Booking from a user's input. Currently it has that each variable must be added. How do I make certain variables not mandatory? i.e. accept a blank value?

```
public static Booking build() throws BuilderException {
    if (date == null)
        throw new BuilderException("Valid date not found");

    if (hours < MINHOURS)
        throw new BuilderException("Minimum hours for booking is " + MINHOURS);

    if (attendance < MINATT)
        throw new BuilderException("Minimum headcount is " + MINATT);

    if (location == null)
        throw new BuilderException("Valid location not found");

    if (building == null)
        throw new BuilderException("Valid building not found");

    if (emailAddress == null)
        throw new BuilderException("Valid email address not found");

    if (contactNumber == null)
        throw new BuilderException("Valid contact number not found");

    return new Booking(date, hours, attendance, location, building, emailAddress, contactNumber);
}

```

So for example I want to keep contactNumber mandatory, but if location/building is left blank then that's ok. Thanks