Currently in a mapping you have 2 options for empty values. The first option is the default which results in any empty values/lists not showing up at all in the output.
This is not always the best because the other party might want to see that the list is empty instead of it not being there at all. For cases like this there is the send empty values = yes, as null option. This is a step in the right direction but I would be much more happy if it returned an actual empty list. So instead of "Key”: null it would be nice if I could return "Key”: .