do not add a logger if app logger is empty.

Within `Application.php` a logger is always added to the app by default, only the value of this logger is `null`.
The check against `isset` doesn't stop the DbalLogger of being added even if the app logger is null.

I've added a UnitTest that required the `class_exist` being mocked since `Symfony\Bridge\Doctrine\Logger\DbalLogger` is not within the default test loading and for testing this is needed.
I would rather use ``null !== $app['logger']`` as it is always set