Fix getUTCDate when datepicker has no selected date

This PR fixes #923  (or should fix) 

Only question left, should we return null or current date? My personal preference would be just null as current date is not correct as no date is selected
Should we cache `dates.get(-1)` ?

```javascript
var last_date = this.dates.get(-1);
if (typeof last_date !== 'undefined') {
	return new Date(last_date);
// . . .
```

`get(-1)` it's just a wrapper for `slice(-1)[0]`