empty classes setting

If the following setting is not present the user if congratulated with an error:

```
symfony_cmf_blog
    class: ~
```

The error is:
```
>php app\console cache:clear

  [ErrorException]
  Notice: Undefined index: class in ...\vendor\symfony-cmf\blog-bundle\Symfony\Cmf\Bundle\BlogBundle\DependencyInjection\SymfonyCmfBlogExtension.php line 38
```

The commit handles the case where the 'class' index may not exist.