feature: empty_containers setting support

The fixes for Hubl will be coming in MRs on the packages which will be dependent on this one.. could you test the ensemble meets Hubl's needs please?

• support new setting empty_containers on Model

e.g.

Meta(Model.Meta):
    empty_containers: ['members']

• it will now be serialized only with @id in the nested serializer (/circles/)
• but it will be serialized as normal when it is the parent (/circle-members/), when it is in a nested viewset (/circles/x/members/) or when it is a child of a different container