<asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:ConnectionString %>" 
            EnableCaching="True" 
            SelectCommand="select top 100 * from TABLE1 P where P.Fld1 like '%'+IsNull(@nom,'')+'%' order by 1 ">
            <SelectParameters>
                <asp:QueryStringParameter Name="nom" QueryStringField="N" ConvertEmptyStringToNull="true" />
            </SelectParameters>
</asp:SqlDataSource>

SqlDataSource1.CancelSelectOnNullParameter=False