[dynamodb2] Implement nonzero for Item to consider empty items falsey

Doing `table.get_item(foo='a')` where no item exists matching `foo='a'` returns an empty item. The returned empty item when passed to `bool`  evaluates as `True`:

```
In [23]: i =  comments_table.get_item(item_id='not_in_there')

In [24]: bool(i)
Out[24]: True
```

This is surprising, and it'd be nice to have `i` show up falsey.