code: private void appendNullCriteria(TableAlias alias, PathInfo pathInfo, NullCriteria c, StringBuffer buf)
      {
          appendColName(alias, pathInfo, c.isTranslateAttribute(), buf);
          buf.append(c.getClause());
      }
docs: Answer the SQL-Clause for a NullCriteria
  
  @param alias
  @param pathInfo
  @param c NullCriteria
  @param buf