SqlDataSource Populate dropdownlist with or without parameter in ASP.Net

I use SqlDataSource control to list out search result when user choose a date, but if the date is null, then it will list out all record. How to do the sql command?

SELECT Ref_No as name, Job_Order_ID as value 
FROM Job_Order 
WHERE (Status <> 'JO_Completed') AND (Delivery_Date = @jaDate) 
OR (Status <> 'JO_Completed') ORDER BY Ref_No 

code to generate dropdownlist

<asp:SqlDataSource ID="NewJobOrderDS" runat="server" SelectCommand="SELECT Ref_No as name, Job_Order_ID as value FROM Job_Order WHERE (Status <> 'JO_Completed') AND (Delivery_Date = @jaDate) ORDER BY Ref_No" OnSelecting="NewJobOrderDS_Selecting">
                                  <SelectParameters>
                                        <asp:ControlParameter ControlID="txtJADate2" Name="jaDate" PropertyName="Text" Type="DateTime"/>   
                                   </SelectParameters>
                                </asp:SqlDataSource>