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.