Fix crash where vtkColorTransferFunction has 0 nodes

Thanks to Jean-Christophe Fillion-Robin for pointing this out.

See

https://github.com/Slicer/Slicer/commit/f59a689a0f6d2912538ed72dad16efc70601f0ed#commitcomment-9407867



for reference.

A basic test for vtkColorTransferFunction is added that crashes
prior to this patch and succeeds with this patch.

Tested-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>