Qt simple DateEditor view shows a date despite the underlying value being none

Initially mentioned in #962, to reproduce:

```
from traits.api import Date, HasTraits
from traitsui.api import UItem, View, DateEditor

class Model(HasTraits):

date = Date()

def default_traits_view(self):
return View(
UItem('date', editor=DateEditor())
)

model = Model(date=None)

if __name__ == '__main__':
model.configure_traits()
```

On Mac OSX and Qt, the view shows this:

This is misleading as the underlying value is `None`.