Error: Cannot convert null to DateTime because it is a non-nullable value type in ASP.Net

This is my code,

public DateTime GetStartDateforCriteria(string criteriaid)
    {
        try
        {
            
            DateTime sdate = new DateTime();
            DataSet dset = new DataSet();
            cSqlStr = "select CriteriaId from SMS_Published_Criteria where CriteriaId=@criteriaid";
            dset = SqlHelper.ExecuteDataset(AppDB, CommandType.Text, cSqlStr, new SqlParameter("@criteriaid", criteriaid));
            if (dset != null && dset.Tables.Count != 0 && dset.Tables[0] != null && dset.Tables[0].Rows.Count > 0)
            {
                cSqlStr = " select ValidityStartDate from SMS_Published_Criteria where VersionNumber=(select max(VersionNumber) from SMS_Published_Criteria) and CriteriaId =@criteriaid";
                sdate = (DateTime)SqlHelper.ExecuteScalar(AppDB, CommandType.Text, cSqlStr, new SqlParameter("@criteriaid", criteriaid));
            }
            else
            {
                
            }
            return sdate;
        }
        catch (Exception ex)
        {
            throw;
        }
    }

if dset empty means,i want to set sdate=null in else part.but it shows

"Cannot convert null to 'DateTime' because it is a non-nullable value type".

How can i set sdate=null? can anyone help me?