[DoctrineParamConverter] Handling of null attributes

I've run into an issue with the ParamConverter and how it handles null values when using custom mappings.

I've got an entity that has a property that can be null, and when using the ParamConverter, if a value is not supplied I am getting back one of the entities with a null field, when I was expecting the ParamConverter to return null instead of an object.

Given that the behaviour has been like this since the beginning, I added an option to the DoctrineParamConverter, `strip_null` that will strip any null attributes from the $criteria array, with an accompanied test.