Support filter parameters in Configuration

Based on work done in doctrine/mongodb-odm#908

My understanding is that this makes it easier to setup filters at boot time, instead of having to fetch them from the collection later on.

For added context, the related PR from MongoDB ODM's Symfony bundle is  doctrine/DoctrineMongoDBBundle#255
well, returning ``null`` when you don't have a string makes sense.
However, when getting an array of parameters, not having parameters is better handled by returning an empty array IMO