WIP: [Sprint/PinkPanther](fix): Better handling of 'user not found' state at v1/groups/member. #871

Summary

Not entirely sure how to replicate this, as if you're not logged in the request should return "Not authenticated etc". That said following it down the stack to the endpoint that is passing in a null user GUID, and taking logic from another part of the site for 'null user handling' makes the most sense to me.