Fix U4-2320 - Error on saving User Permissions

Nodes can be null but NodeIds property always returns a collection and this will return Ids from Nodes if _nodeIds are null and Nodes is not.
Merge pull request #6 from dpen2000/UserPermisionsFixIn6.1.2

Fix U4-2320 - Error on saving User Permissions (for 6.1.2 release)