数据为空时，返回值问题

方法(1) private DataTable getMainChildTable(DataTable dt,out int maxNum)
{
 DataTable table = new DataTable();
        table.Columns.Add("月份", typeof(String));
        table.Columns.Add("年龄阶段",typeof(string));
        table.Columns.Add("月内增加值", typeof(Double));

        maxNum = 0;
        foreach (DataRow drr in dt.Rows)
        {
            DataRow dr = table.NewRow();
            dr["月份"] = "";
            dr["年龄阶段"] = "";
            dr["月内增加值"] = 0;

            if (int.Parse(drr["allcount"].ToString()) > maxNum)
            {
                maxNum = int.Parse(drr["allcount"].ToString());
            }

            switch (drr["BirthdayOption"].ToString())
            {
                case "30岁以下":
                    dr["月份"] = drr["M"]+"月";
                    dr["年龄阶段"] = "30岁以下";
                    dr["月内增加值"] = drr["allcount"];
                    break;
                case "30-60岁":
                    dr["月份"] = drr["M"]+"月";
                    dr["年龄阶段"] = "30-60岁";
                    dr["月内增加值"] = drr["allcount"];
                    break;
                case "60岁以上":
                    dr["月份"] = drr["M"]+"月";
                    dr["年龄阶段"] = "60岁以上";
                    dr["月内增加值"] = drr["allcount"];
                    break;
                case "未知年龄":
                    dr["月份"] = drr["M"]+"月";
                    dr["年龄阶段"] = "未知年龄";
                    dr["月内增加值"] = drr["allcount"];
                    break;
            }
            table.Rows.Add(dr);
        }
        return table;
}

方法 (2)private DataTable getMainTable(out int maxNum)
{
    string startDate = txtCheckDateStart.Value;
   string organizationId = manageMHRA.SelectedCategoryId;

        StringBuilder bStrSql = new StringBuilder();
        bStrSql.Append("select ");
        bStrSql.Append(" Case when DATEDIFF(YEAR,Bithday,GETDATE())<30 then '30岁以下'  ");
        bStrSql.Append(" when DATEDIFF(YEAR,Bithday,GETDATE())>30 And DATEDIFF(YEAR,Bithday,GETDATE())<60 then '30-60岁' ");
        bStrSql.Append(" when DATEDIFF(YEAR,Bithday,GETDATE())>60  then '60岁以上' else '未知年龄' end BirthdayOption ,COUNT(0) allcount, ");
        bStrSql.Append(" SUBSTRING(convert(char(10),TIH.IllDate,112) ,0,7) as M  ");
        bStrSql.Append(" from T_IllOldHistory TIH inner join T_FamilyMember ");
        bStrSql.Append(" on TIH.MemberId=T_FamilyMember.MemberId where 1=1 and  ClassifyId= 'b25f19ad5fd54b4eb88e0444c1f50400' and BlDefinite=1 ");

        if (!string.IsNullOrEmpty(startDate))
        {
            bStrSql.AppendFormat(" and datediff(year,TIH.IllDate,'{0}')=0", DateTime.ParseExact          (startDate, "yyyy", System.Globalization.CultureInfo.CurrentCulture).ToString("yyyy-MM-dd"));
        }
        if (!string.IsNullOrEmpty(organizationId))
        {
            bStrSql.AppendFormat(" and Exists (select 1 from OrganizationCategory WHERE T_FamilyMember.OCategoryId = OrganizationCategory.CategoryId AND CategoryNumber LIKE (Select CategoryNumber + '%' FROM OrganizationCategory WHERE CategoryId='{0}' and DeletionStateCode=0)) ", organizationId);
        }

        bStrSql.Append(" group by SUBSTRING(convert(char(10),TIH.IllDate,112) ,0,7) , Case when DATEDIFF(YEAR,Bithday,GETDATE())<30 then '30岁以下'  when DATEDIFF(YEAR,Bithday,GETDATE())>30 And DATEDIFF(YEAR,Bithday,GETDATE())<60 then '30-60岁' ");
        bStrSql.Append(" when DATEDIFF(YEAR,Bithday,GETDATE())>60  then '60岁以上' else '未知年龄' end order by M ASC,BirthdayOption asc");

        DataTable dt = GSP.(bStrSql.ToString());//在这如果dt为空时，怎样不让它报错（如果没有数据出现溢出错误）？？？

        return getMainChildTable(dt, out maxNum);//调用方法(一)
}

方法(2)返回getMainChildTable(DataTable ,out maxNum)

当方法二中的数据为空时想提示用户以下(不让它报错)