GroupManager#getGroups should allow guest user calls

GroupManager#getGroups currently doesn't allow calls with a null value for the member parameter. However we do use null everywhere for guest user references, and it's an easy mistake to have an API manipulating user references to accept null values and to rely on the GroupManager API.

So I believe it would be a nice little improvment to immediately return an empty collection when calling that API with null value, instead of getting a NPE.