Forms Data Controls :: A Required Input Field DropDownList If Old Records Have Null In This Field?

I have a DetailsView on the page and I have made one of the fields a TemplateField. In the EditItemTemplate and InsertItemTemplate I have a DropdownList that is databound to a table in my Sql Server database. I wanted to add an initial value to the DropdownList
namely "- select -" .

I set the AppendDataBoundItems property to true and added the initial value as a ListItem.Markup of the DropdownList

<asp:DropDownList ID="DropDownList_HP" runat="server"

AppendDataBoundItems="True" DataSourceID="SqlDataSource_HP" [code]...

This works fine for new input. The problem is however that the database already has records that were entered through a Windows Application and many of these records has a null value in this field and exceptions are thrown when I tried to open these records
and the system tried to set the SelectedValue of the DropdownList.After some more searching I found this help

http://msdn.microsoft.com/en-us/library/ms366709.aspx

So I changed the ListItem in the DropDownList markup to the following:

<asp:DropDownList ID="DropDownList_RefHospDV4" runat="server"

AppendDataBoundItems="True" DataSourceID="SqlDataSource_RefHosp" [code]...

This now solved the problem of opening a record where the value is null in the database, BUT now the RequiredFieldValidator is not validating anymore to make sure that a databound item is selected for this field and not the initial value "- select -". So basically now it is not checking anymore to see if valid input has been entered for the DropDownList and it accepts "- select -" thus it acts as if the field is not a required field anymore.

In short what is required is that I want to make sure that the user enters a valid selection in the DropDownList, but it must also cater for old records that do not have this field entered yet so that those old records can be opened in the DetailsView .Opening Old records (with null in that field):When these old records are opened in the DetailsView the DropDownlist should show "- select -" when the value in the database is null.

Saving records (old or new records):When saving the record in Insert mode or Update mode the RequiredFieldValidator should show that a valid input is not selected if the DropDownList is still on "- select -".