Insert NULL in DataBase if user does not select Item from DropDownList in ASP.Net

Hi

I have some textbox and Dropdownlist in my page and bind it from database

 BindDropDownList(DDL3, "Guidcenter", "Centername","id");

 DDL3.Items.Insert(0, new ListItem(" please select ", "0"));

I have button in my page that when user click on it insert data into database

here when users didn't select Item from dropdownlist it insert "please select" ( text that I define for DDL ) into database but I want when users didn't select Item from database it insert NULL in database

How i can do it?

Thanks alot

protected void ImageButton2_Click1(object sender, ImageClickEventArgs e)
    {
        string data = Server.UrlDecode(Request.QueryString["BehCode"]);

        SqlCommand _cmd = new SqlCommand("insertinfo", _cn);
        _cmd.CommandType = CommandType.StoredProcedure;
        _cn.Open();

            _cmd.Parameters.AddWithValue("@Name", txtstore.Text);
            _cmd.Parameters.AddWithValue("@Ownername", Txtname.Text);

            _cmd.Parameters.AddWithValue("@ShoppingCenter", DDL3.SelectedItem.Text);
            _cmd.Parameters.AddWithValue("@behcode", data);

            _cmd.ExecuteNonQuery();
            Session["Message"] = true;

            Response.Redirect(Request.Url.AbsoluteUri);
  
        _cn.Close();
      
    }

ALTER procedure [dbo].[insertinfo]
@behcode nvarchar(15),
@Name nvarchar(30),@Ownername nvarchar(30),@ShoppingCenter nvarchar(50)
as
begin

update House_info set Name=@Name,Ownername=@Ownername,BehCode=@Behcode
ShoppingCenter=@ShoppingCenter
where BehCode=@Behcode 

end