Support loans without user ID and circulation interface 4.0

In order to allow for anonymization of loans it needs to be possible to disassociate a loan with a user, which means that clients of the circulation interface version 4.0 need to support loans that do not have userId properties

This should be dual interface support for 3.x and 4.x where possible. If this is not possible, please hold back merging until the changes can be coordinated (so that both reference environments can construct cohesive backends)