code: 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";
      }
    }
docs: /*
  @see org.criteria4jpa.criterion.Criterion#toQueryString(org.criteria4jpa.Criteria, org.criteria4jpa.impl.CriteriaQueryBuilder)