A style sheet just makes these types of things easier so you can go back and make a single change rather than having to dig back into your code. To see this in action, open up the Web Sample Application that comes with StrataFrame. Go to the SearchCustomers.aspx page and there is a ListView on that page that uses the HeaderCssClass tag, CssClass tag, and populates one of the columns through the RowPopulating event. I think that this will get you going in the right direction.
If you have any 3rd party web controls such as Infragistics that have a web based date time control, you can also set the "NULL" value to be 1/1/1800 which would show an empty field.
Last, you could create your own text box which and implement your own data binding and NULL value support. The NULL value in this case would be 1/1/1800 which you could then return an empty string when displaying the control.