Fixed DoctrineServiceProvider when no logger

When the provider is used with `Pimple` alone, it throws an exception if the `logger` is not defined.

It was previously working, but it seems to be introduced in https://github.com/silexphp/Silex/commit/784a24665de285cb8c31db2c12c4785eb5605773.
I think it does not work due to the way `offsetExists` is implemented in `Pimple`

```php
$pimple = new Container();
$pimple['test'] = null;
$this->assertTrue(isset($pimple['test'])); // This is true
```