code: @Override
      public SqlInfo buildSqlInfo(BuildSource source) {
          Node node = source.getNode();
  
          /* 获取field和match属性,如果匹配中'match'属性没有值或者值为真，则认为是必然生成项. */
          String fieldText = XmlNodeHelper.getAndCheckNodeText(node, ZealotConst.ATTR_FIELD);
          String matchText = XmlNodeHelper.getNodeAttrText(node, ZealotConst.ATTR_MATCH);
          Boolean matchValue = (Boolean) ParseHelper.parseExpressWithException(matchText, source.getParamObj());
          if (StringHelper.isBlank(matchText) || Boolean.TRUE.equals(matchValue)) {
              return XmlSqlInfoBuilder.newInstace(source).buildIsNullSql(fieldText);
          }
  
          return source.getSqlInfo();
      }
docs: 构建" IS NULL "和" IS NOT NULL "查询的动态条件sql.
  @param source 构建所需的资源对象
  @return 返回SqlInfo对象