空字段显示

我想把数据库中sub_title的字段为空的数据，在前台页面显示时，用 资讯 来代替 一下是我的代码 请大家看看对不 
public DataTable GetProductRelationArticle(int num)
        {
            DataTable dt_article = new DataTable();            dt_article.Columns.Add("articleid", Type.GetType("System.Int32"));
            dt_article.Columns.Add("sub_title", Type.GetType("System.String"));//2010-06-02添加
            dt_article.Columns.Add("title", Type.GetType("System.String"));
            dt_article.Columns.Add("ArticleUrl", Type.GetType("System.String"));
            dt_article.Columns.Add("PostDate", Type.GetType("System.DateTime"));            #region 从mysql新发布系统中获取设置关联的产品文章
            using (SqlConnection mysqlconn = new SqlConnection(BasePage.articleMySQLConnString))
            {
                try
                {
                    mysqlconn.Open();
                    string mysqlstr = "SELECT top {0} id AS articleid,sub_title,title,url AS ArticleUrl,publish_time AS PostDate " +
                        "FROM article  ORDER BY publish_time DESC ";
                    mysqlstr = string.Format(mysqlstr,  num * 2);
                    //PCommon.WriteLog(mysqlstr);
                    DataSet mysqlDs = SqlHelpers.ExecuteDataset(mysqlconn, mysqlstr);
                    mysqlconn.Close();                    DataTable dt_article1 = mysqlDs.Tables[0];
                    DataRow dr = dt_article1.NewRow();
                    if (string.IsNullOrEmpty(dr["sub_title"].ToString()))
                    {
                        dr["sub_title"] = "资讯";
                    }
                    dt_article1.Rows.Add(dr);
                    dt_article.Merge(dt_article1);
                   
                }
                catch (Exception ex)
                {
                    //PCommon.WriteLog("mysql Query:\r\n" + ex.ToString());
                }
                mysqlconn.Close();
            }
            #endregion
            dt_article.AcceptChanges();            DataTable dt_result = dt_article.Clone();
            foreach (DataRow dr in dt_article.Rows)
            {
                if (num-- < 1) break;
                //过滤已经存在的文章
                if (dt_result.Select(string.Format("title='{0}'", dr["title"].ToString().Replace("'", "''"))).Length > 0)
                {
                    continue;
                }
                dt_result.Rows.Add(dr.ItemArray);
            }
            dt_result.AcceptChanges();            return dt_result;
        }