I am using a CellTable with also DatePickerCell and I'd like to be able to display also dates that are not set, e.g. "empty" date. But in implementing getValue method I have to return something:

```
        Column<EventProxy, Date> startTimeColumn = new Column<EventProxy, Date>(
            new DatePickerCell()) {
                @Override
                public Date getValue(EventProxy object) {
                    Date ret = object.getStartTime();
                    return ret != null ? ret : new Date();
                }
    };

```

If object.getStartTime() is null, it means that it is not set and I want to display it as such. Or with empty label or with text "empty". As I've said, method getValue has to return something. If I return null, I get exceptions later, if I return concrete date it displays it as valid date. Is there any other option? Some special date tag or object that DatePickerCell would recognize as empty or unset value?