Fix for empty keys in app.php are not replaced by artisan key:generate

In Laravel 3, it was required that the app.key property be an empty string for replacement to happen with artisan key:generate

eg. 'key' => '',

When running artisan key:generate with an empty key in Laravel 4 this does not happen.  In addition an inaccurate message stating that the application key was set successfully is output by this command.