Handle Yandex Response with No City or State

If I try to find some location without city in yandex it give an error:

```
geocode 57.423359 55.892596 -s yandex 
ruby-1.9.3-p327/gems/geocoder-1.1.6/lib/geocoder/results/yandex.rb:20:in `city': undefined method `[]' for nil:NilClass (NoMethodError)
```

It happpens because for some regions Yandex not provide the city or town.

Now if city not presented empty string would return - "".