@Override
    public void write(AssignValueUpdate update, DataType superType) {
        if (update.getValue() == null) {
            putByte(null, (byte) 0);
        } else {
            putByte(null, (byte) 1);
            writeValue(this, superType, update.getValue());
        }
    }
Write out the value of weighted set field

@param field - field description (name and data type)
@param ws    - field value
document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer6.java