how do I pass NULL selected values to stored procedure ???

I have 3 dropdown lists on search.aspx.  My question is if the user does not select
a value how do I pass null in the query string and in code behind pass that to Stored Procedure.
For example say its a Country DDL. If they do not select a country and leave it null I want to pass in Select ALL
to the stored procedure.
I put all the code below

in code behind I pass the selected values in the query string to Results.aspx

 protected void Unnamed1_Click(object sender, EventArgs e)     {         Response.Redirect("SearchForModels1.aspx?AssignmentId=" + D4.SelectedItem.Value.ToString()             + "&CountryId=" + D3.SelectedItem.Value.ToString() + "&EthnicityId=" + D2.Selec ...