code: public ZealotKhala orIsNull(String field) {
          return this.doIsNull(ZealotConst.OR_PREFIX, field, true,true);
      }
docs: 生成带" OR "前缀" IS NULL "的SQL片段.
  <p>示例：传入 {"a.name"} 参数，生成的SQL片段为：" OR a.name IS NULL "</p>
  
  @param field 数据库字段
  @return ZealotKhala实例