FELIX-4399, FELIX-4400: UserAdmin MongoDB store:

- when creating a new role, we should return the created role if we
  have good indications that the creation was successful;
- MongoDB returns null for empty properties, causing NPEs on several
  occassions.



git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1560612 13f79535-47bb-0310-9956-ffa450edef68