Fix crash of vtkPointSetCellIterator
When vtkPointSetCellIterator gets a vtkPointSet with a NULL vtkPoints member it would crash because it would try to set the vtkPointSet datatype. Now this is avoided if the vtkPointSet has a NULL vtkPoints.
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When vtkPointSetCellIterator gets a vtkPointSet with a NULL vtkPoints member it would crash because it would try to set the vtkPointSet datatype. Now this is avoided if the vtkPointSet has a NULL vtkPoints.