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);
}