title: Services not loaded at Resources/config/services.yml, why?
body: <p>I'm running a issue here and I don't know why or where this is failing, perhaps I miss some configuration or so, anyway, I have this code at <code>DependencyInjection\AppExtension.php</code> file:</p>
  
  <pre><code>use Symfony\Component\DependencyInjection\ContainerBuilder;
  use Symfony\Component\Config\FileLocator;
  use Symfony\Component\HttpKernel\DependencyInjection\Extension;
  use Symfony\Component\DependencyInjection\Loader;
  
  class AppExtension extends Extension
  {
      /**
       * {@inheritdoc}
       */
      public function load(array $configs, ContainerBuilder $container)
      {
          $configuration = new Configuration();
          $config = $this-&gt;processConfiguration($configuration, $configs);
  
          $loader = new Loader\YamlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
          $loader-&gt;load('services.yml');
      }
  }
  </code></pre>
  
  <p>Then at <code>Resources/config/services.yml</code> I have this:</p>
  
  <pre><code>services:
      pdone.twig.extension:
          class: GroupDCA\PDOneBundle\Extension\PDOneTwigExtension
          tags:
              -  { name: twig.extension }
  </code></pre>
  
  <p>For some reason isn't working. This mean I got this error:</p>
  
  <blockquote>
    <p>The filter "empty" does not exist in PDOneBundle::pdone.html.twig at line 1 </p>
  </blockquote>
  
  <p>Now if I move the services definition to <code>config/config.yml</code> I got this error instead:</p>
  
  <blockquote>
    <p>Compile Error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)</p>
  </blockquote>
  
  <p>Which makes me think the bundle is not going through DependecyInjection, what I am missing here? Why different errors?</p>