Early return in vtkSMPVRepresentationProxy::GetLUTProxy to avoid volume rendering warning

In 6136acc0 a change was introduced that made calling vtkSMPVRepresentationProxy::SetScalarBarVisibility with a nullptr view throw an error (only when using mesa?).

This MR implements a minimal fix by exiting the method preemptively when view is a nullptr effectively reverting a portion of the above mentionned commit.