Fix vtkGPUVolumeRayCastMapper multi-volume-rendering crash

After removing a volume from the input vtkMultiVolume and then updating another input volume,
vtkGPUVolumeRayCastMapper crashed in vtkGPUVolumeRayCastMapper::CloneInput because
input was nullptr and it was dereferenced when calling input->GetMTime().

Fixed by checking input for nullptr.

Also updated vtkGPUVolumeRayCastMapper::TransformInput so that it clears the transformed input
when input is nullptr.