A constraint that tests if a stepfield value is null.
This corresponds to IS NULL in SQL SELECT-syntax.
author: Rob van Maris version: $Id: FieldNullConstraint.java,v 1.2 2003/03/10 11:50:47 pierre Exp $ since: MMBase-1.7
Returns a string representation of this FieldNullConstraint.
The string representation has the form
"FieldNullConstraint(inverse:<:inverse>, field:<field>,
casesensitive:<casesensitive>)"
where