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.
Maybe use `isset` instead ? That checks if the key is defined in the array AND the value is not NULL.