[DDC-2987] Enable empty prefixes for inlined embeddable

This fixes [DDC-2987]

This makes it possible to map a field from an embeddable to a database field with the same name, without any prefix added.

Example:
- an embeddable object "Id" with a property "id"
- per default this would map inline to id_id
- supplying null or '' as columnPrefix does not work due to the ! empty() check
- with my little change, if columnPrefix : false is supplied in the mapping config this will now map to a db column "id"

To build Ids as ValueObjects is a very common approach in DDD, so ihmo this is a must have.