BUG: Accept qMRMLSceneModel::updateNodeItems with node with no scene

To have no scene in qMRMLSceneModel::updateNodeItems()) is a valid case.
When closing the scene, the "Close" state is popped and SceneClosedEvent
event is triggered, caught by an observer that eventually modify a
node, such modification is caught by a scene model that has not yet
received teh SceneClosed event. It asks if the scene is closing but it
is no longer (see vtkMRMLScene::EndState())

Closes #3513

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23028 3bd1e089-480b-0410-8dfb-8563597acbee