Needs Help With Adding an If Statement

Please look at the code below. Please look at the bolded line of code. I'd like to add some code to let an event show up if the EventStartDate has a value and the EventEndDate doesn't have a value. For example, there may be some one day events. I am not sure how to do this. Could someone please help me with this? I got the following error message when there was just an one day event with the EventEndDate field null.

strSQL = " SELECT * FROM EVENTS WHERE (EventStartDate >= '" & datCalendarStart.ToString()  & "' " & _ 
" AND EventStartDate <= '" & datCalendarEnd.ToString()  & "') OR (EventEndDate >= '" & datCalendarStart.ToString()  & "' " & _
" AND EventEndDate <= '" & datCalendarEnd.ToString()  & "') OR (EventStartDate <= '" & datCalendarStart.ToString()  & "' " & _
" AND EventEndDate >= '" & datCalendarEnd.ToString()  & "') ORDER BY EventStartDate "

        myCommand = New SqlCommand( strSql, Conn)
       
        Conn.Open()
 
        myReader = myCommand.ExecuteReader()

        lblDates.Text = " <P> Events for the month of " & calCalendar.SelectedDate.ToString("MMMM yyyy") & "<br> <br>"

        ' Always call Read before accessing data.
        While myReader.Read()
            
           lblDates.Text &= myReader.GetDateTime(2).ToString("MM/dd/yy") & " - " & myReader.GetDateTime(3).ToString("MM/dd/yy") & "<br>"