Update MapQuestProvider.php

MapQuest is not returning any useful information for coordinates in London.  However, because it does include a locations element in the returned JSON, a results array is returned from the provider, with null values for all fields except the given coordinates.

Because a (bad) result is returned, the next provider in my chain is never queried to get the correct data.

Modified the provider to only return results if there are any non-empty values to be written.

The test file I was using to verify the behavior is below:

```
<?php

require_once dirname(dirname(__FILE__)).'/vendor/autoload.php';

header('Content-type: text/plain');

$geocoder = new \Geocoder\Geocoder();
$buzz = new \Buzz\Browser(new \Buzz\Client\Curl());
$adapter = new \Geocoder\HttpAdapter\BuzzHttpAdapter($buzz);
$chain = new \Geocoder\Provider\ChainProvider(
    array(
            new \Geocoder\Provider\MapQuestProvider($adapter, 'mapquest_api_key'),
            new \Geocoder\Provider\GoogleMapsBusinessProvider($adapter, 'google_client_id', 'google_private_key')
    )
);
$geocoder->registerProvider($chain);

$coordinatesArray = array(
        array('38.8953003', '-77.0328011'), // Washington, DC
        array('48.868865', '2.339355'), // Paris
        array('51.507276', '-0.12766') // London
);
foreach ($coordinatesArray as $coordinates) {
    $geocoded = $geocoder->reverse($coordinates[0], $coordinates[1]);
    echo "reverse geocoding $coordinates[0], $coordinates[1]:\n";
    print_r($geocoded->toArray());
}
```

Output before patch:
```
38.8953003, -77.0328011:
Array
(
    [latitude] => 38.8953
    [longitude] => -77.032801
    [bounds] => 
    [streetNumber] => 
    [streetName] => Pennsylvania Avenue Northwest
    [zipcode] => 20004
    [city] => Washington
    [cityDistrict] => 
    [county] => District Of Columbia
    [countyCode] => 
    [region] => 
    [regionCode] => 
    [country] => Us
    [countryCode] => 
    [timezone] => 
)
48.868865, 2.339355:
Array
(
    [latitude] => 48.868865
    [longitude] => 2.339355
    [bounds] => 
    [streetNumber] => 
    [streetName] => 5 Rue Des Filles Saint-Thomas
    [zipcode] => 75002
    [city] => Paris
    [cityDistrict] => 
    [county] => Paris
    [countyCode] => 
    [region] => Ile-De-France
    [regionCode] => 
    [country] => Fr
    [countryCode] => 
    [timezone] => 
)
51.507276, -0.12766:
Array
(
    [latitude] => 51.507276
    [longitude] => -0.12766
    [bounds] => 
    [streetNumber] => 
    [streetName] => 
    [zipcode] => 
    [city] => 
    [cityDistrict] => 
    [county] => 
    [countyCode] => 
    [region] => 
    [regionCode] => 
    [country] => 
    [countryCode] => 
    [timezone] => 
)
```

As you can see, the result for London is all blank except for the given coordinates.

Output after patch:
```
38.8953003, -77.0328011:
Array
(
    [latitude] => 38.8953
    [longitude] => -77.032801
    [bounds] => 
    [streetNumber] => 
    [streetName] => Pennsylvania Avenue Northwest
    [zipcode] => 20004
    [city] => Washington
    [cityDistrict] => 
    [county] => District Of Columbia
    [countyCode] => 
    [region] => 
    [regionCode] => 
    [country] => Us
    [countryCode] => 
    [timezone] => 
)
48.868865, 2.339355:
Array
(
    [latitude] => 48.868865
    [longitude] => 2.339355
    [bounds] => 
    [streetNumber] => 
    [streetName] => 5 Rue Des Filles Saint-Thomas
    [zipcode] => 75002
    [city] => Paris
    [cityDistrict] => 
    [county] => Paris
    [countyCode] => 
    [region] => Ile-De-France
    [regionCode] => 
    [country] => Fr
    [countryCode] => 
    [timezone] => 
)
51.507276, -0.12766:
Array
(
    [latitude] => 51.5072997
    [longitude] => -0.1275094
    [bounds] => Array
        (
            [south] => 51.5072997
            [west] => -0.1275094
            [north] => 51.5072997
            [east] => -0.1275094
        )

    [streetNumber] => 3
    [streetName] => Whitehall
    [zipcode] => SW1A 2EL
    [city] => London
    [cityDistrict] => 
    [county] => Greater London
    [countyCode] => GT LON
    [region] => 
    [regionCode] => 
    [country] => United Kingdom
    [countryCode] => GB
    [timezone] => 
)
```

Here, the MapQuest provider was skipped and the correct result was returned from the GoogleMapsBusinessProvider.