Fix empty DTSTART

If you try expand calendar object, with simple event like:
```
BEGIN:VEVENT
DTSTAMP:20130101T000000Z
UID:xxxxxxx-xxx-xxxx-xxxxxxx
END:VEVENT
```
you get error `Call to a member function getDateTime() on a non-object in ...lib/Sabre/VObject/Component/VEvent.php:52`

This commit fix it.