Querystring nulla e valore di default

    private void popola()
    {

        using (OdbcConnection conn = new OdbcConnection(connectionString))
        {

            OdbcCommand objCommand = new OdbcCommand();

            string SQL = String.Empty;
            string spSezione = string.Empty;

            if (Request.QueryString["sezione"] != null)
                spSezione = Request.QueryString["sezione"].ToString();

            if (Request.QueryString["sezione"] != string.Empty)
            {
                SQL = "SELECT * FROM news WHERE _stato='1' and _sezione=? ORDER BY _data DESC;";
                objCommand.Parameters.AddWithValue("?", spSezione);
            }
            else
            {
                SQL = "SELECT * FROM news WHERE _stato='1' ORDER BY _data DESC;";
            }

            conn.Open();
            objCommand.Connection = conn;
            objCommand.CommandType = CommandType.Text;
            objCommand.CommandText = SQL;

            DataSet ds = new DataSet();
            OdbcDataAdapter da = new OdbcDataAdapter(objCommand);
            da.Fill(ds);
            MyGrid.DataSource = ds;
            MyGrid.DataBind();

            ds.Dispose();
            da.Dispose();
            conn.Close();
        }
    }