BUG: Fix crash with vtkMRMLSelectionNode and vtkSlicerUnitsLogic

Since r23192, the GetUnitNodes() can return empty nodes.
This triggered asserts in vtkSlicerUnitsLogic.
vtkSlicerUnitLogic is only interested in unit quantities and IDs.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23222 3bd1e089-480b-0410-8dfb-8563597acbee