Account for a WP_Error result from Google Suggest request

`PHP message: PHP Fatal error: Cannot use object of type WP_Error as array in /var/www/wp-content/plugins/wordpress-seo/admin/ajax.php on line 96`

If a request fails and the result is a WP_Error object, a fatal error is thrown when attempting to access the result as an array.

This wraps the processing of the response in a check for the error and results in an empty return array if one is found.

It may be beneficial to provide some sort of error message on the front end.