Handling Null Fields With Three-tier Architecture

I using Visual Web Designer Express (with Visual Basic), with a SQL Server 2000 database.   I have a prototype application running satisfactorily using code that builds queries, but I'm now trying to rebuild the application "properly" using three-tier architecture.  I have been following the principles of Scott Mitchell's tutorials.  I have created an database .xsd with a table adaptor, and replaced the rather grotty query-building code in the business layer with better code referencing the table adaptor.   Thus where the first version had code: -

        Dim queryString As String = "SELECT * FROM NZGDB_User WHERE USRid = '" & Userid & "'"
        Dim message As String = ""
        Dim Found As Boolean = False
        Try
            Using connection As New SqlConnection(GDB_AppSettings.Connection)
                Dim command As New SqlCommand(queryString, connection)
                connection.Open()

                Dim reader As SqlDataReader = command.ExecuteReader()

                If reader.Read() Then
                    Found = True
                    _USRid = reader("USRid")
                    _USRPassword = reader("USRPassword")
                    _USREmail = reader("USREmail")
                    _USRTitle = reader("USRTitle")
                    _USRName = reader("USRName")
                    _USRRole = reader("USRRole")
                    If IsDBNull(reader("USRAgreedConditions")) = False Then
                        _USRAgreedConditions = reader("USRAgreedConditions")
                    End If
                End If
                reader.Close()
            End Using
        Catch ex As Exception
            If Left(Err.Description, 68) = "An error has occurred while establishing a connection to the server." Then
                Return "Cannot open database to logon"
            Else
                Return Err.Description
            End If
        End Try

the new version is much more elegant: -

        Dim taUser As New NZGDBTableAdapters.NZGDB_UserTableAdapter()

        Dim tbUser As NZGDB.NZGDB_UserDataTable = taUser.GetUserByUserid(userid)
        If tbUser.Count <> 1 Then   '   Anything other than 0 or 1 should be impossible
            Return "User not found"
        End If

        Dim trUser As NZGDB.NZGDB_UserRow = tbUser(0)
        _USRid = userid
        _USRPassword = password
        _USREmail = trUser.USREmail
        _USRTitle = trUser.USRTitle
        _USRName = trUser.USRName
        _USRRole = trUser.USRRole
        _USRAgreedConditions = trUser.USRAgreedConditions

However, there is a problem.  The database field USRAgreedConditions is a Datetime field that can be null.  The new version works perfectly when it is NOT null, but throws an exception: -

System.Data.StrongTypingException was unhandled by user code
  Message="The value for column 'USRAgreedConditions' in table 'NZGDB_User' is DBNull."
  Source="App_Code.wt2vzoc1"
  ....

There is no point in writing: -
        If Not IsDBNull(trUser.USRAgreedConditions) Then
            _USRAgreedConditions = trUser.USRAgreedConditions
        End If
because the exception occurs within the automatically-created code in the data access layer.  I tried changing the Nullvalue property of the field USRAgreedConditions in the table adaptor, but the only valid option is (Throw Exception) unless the field is a String.  Of course USRAgreedConditions is a Datetime field, so I can't change the property.

It seems that my only options are: -
1.   To stick with the old query-building code.   But this doesn't give me the advantages of a proper 3-tier architecture
2.   To change the generated code in wt2vzoc.  This MUST be a bad idea - surely I should leave this code untouched.  Besides, what if the table adaptor has to be regenerated when I change the table design?
3.   Code a Try block within the business layer: -
    Try
         _USRAgreedConditions = trUser.USRAgreedConditions
    Catch ex As Exception
         _USRAgreedConditions = Nothing
    End Try

This seems to work OK, but seems less elegant than the original code in the old version: -
       If IsDBNull(reader("USRAgreedConditions")) = False Then
            _USRAgreedConditions = reader("USRAgreedConditions")
       End If

Is there a better way?