Set Custom Date Column on Ultragrid as Null


I have added 3 columns to the Time Phase form that show Part Number, Quantity and Week Commencing for any record related to a Job Number.  However, as many records are Purchase Suggestions, etc I want the Week Commencing to show null if no related record but I am struggling to show null for a date field.  As you can see from the code below I have blanked the row that I am struggling with from the code.  We are on Vantage 8.03.409a - any help appreciated.  Thanks!

    Private Function getWeekComm(ByVal job As String) As String
        Dim weekComm As DateTime
        Dim recordSelected As Boolean
        Dim showSearch As Boolean = False
        Dim whereClause As String = "JobNum = '" + job + "'"
        Dim dsJobInfo As DataSet = SearchFunctions.listLookup(TimePhasForm,"JobAsmSearchAdapter", recordSelected, showSearch, whereClause)
            If recordSelected Then
                weekComm = dsJobInfo.Tables(0).Rows(0)("StartDate").AddDays((dsJobInfo.Tables(0).Rows(0)("StartDate").DayOfWeek - DayOfWeek.Monday) * -1)
                'weekComm = ""
            End If

        Return weekComm

    End Function