src/main/java/org/criteria4jpa/criterion/IsNullExpression.java
public String toQueryString(Criteria criteria, CriteriaQueryBuilder queryBuilder) {
    String path = queryBuilder.getAbsolutePath(criteria, propertyName);
    if( !negate ) {
      return path+" IS NULL";
    } else {
      return path+" IS NOT NULL";
    }
  }
public String toQueryString(Criteria criteria, CriteriaQueryBuilder queryBuilder) {
    String path = queryBuilder.getAbsolutePath(criteria, propertyName);
    if( !negate ) {
      return path+" IS NULL";
    } else {
      return path+" IS NOT NULL";
    }
  }
/*
@see org.criteria4jpa.criterion.Criterion#toQueryString(org.criteria4jpa.Criteria, org.criteria4jpa.impl.CriteriaQueryBuilder)