fix for yandex city fetching: case of no locality name

Sometimes we are getting back results with ``Locality`` key in ``SubAdministrativeArea``, but that ``Locality`` has no ``LocalityName``. Instead, it has only ``DependentLocality`` and ``DependentLocalityName`` keys, e.g.:
```ruby
"GeoObject":{
    "metaDataProperty":{
        "GeocoderMetaData":{
            "kind":"house",
                "text":"Canada, Quebec, Gatineau, Hull, Rue Hormidas-Dupuis, 14",
                "precision":"exact",
                "AddressDetails":{
                    "Country":{
                        "AddressLine":"Quebec, Gatineau, Hull, Rue Hormidas-Dupuis, 14",
                        "CountryNameCode":"CA",
                        "CountryName":"Canada",
                        "AdministrativeArea":{
                            "AdministrativeAreaName":"Quebec",
                            "SubAdministrativeArea":{
                                "SubAdministrativeAreaName":"Gatineau",
                                "Locality":{
                                    "DependentLocality":{
                                        "DependentLocalityName":"Hull",
                                        "Thoroughfare":{
                                            "ThoroughfareName":"Rue Hormidas-Dupuis",
                                            "Premise":{
                                                "PremiseNumber":"14"
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
        }
    }
}
```
According to bugtrackers of geocoding services, this is a known issue:
http://code.google.com/p/gmaps-api-issues/issues/detail?id=3320

Here we fix a problem with raise of ``undefined method 'empty' for nil class`` when asking a result object for its city:
```ruby
    def city
      if state.empty? and address_details.has_key? 'Locality'
        address_details['Locality']['LocalityName']
      elsif sub_state.empty? and address_details['AdministrativeArea'].has_key? 'Locality'
        address_details['AdministrativeArea']['Locality']['LocalityName']
     # !!! 
     # NOTE: this could raise an error due to nil state of sub_state_city
     # !!!
      elsif not sub_state_city.empty?
        sub_state_city
      else
        ""
      end
    end
```