this implementation of the FieldConversion interface converts
int to Integer, Integer to int. If the int is zero, then
we don't have a value for the key and it must be set to null, not
zero.
author: Aaron Oathout version: $Id: Int2IntegerFieldConversion.java,v 1.5.2.1 2005/12/21 22:23:38 tomdz Exp $