original title: fallback to default implementation when class property is empty or wrongly set ?

To be discussed: as per MAGNOLIA-1688, it might be useful to fallback to the default implementation class when a class property is set to an unexisting class or is empty. I am not entirely convinced, because this is typically a configuration mistake and it should not be ignored. Maybe a precise warning/error log before falling back would be good. (depending on what component is being configured, this could otherwise block the system - i.e for filters for instance)