BUGFIX: singleton('Group')->Members() fails

Running Members() on a Group that has no Db record causes UnsavedRelationList to be returned by DirectMembers() which in turn causes alterDataQuery() to fall over when called on an UnsavedRelationList. This just adds a simple check to prevent it.
Merge pull request #1979 from nedmas/patch-1

BUGFIX: singleton('Group')->Members() fails