BUG: Fix Slicer crash on DTIVolume reload after scene is closed.

Handle the case when newVolumeDispalyNode is NULL in setMRMLVolumeNode().
Fixes #3380

From: Yuzheng Zhou <yuzheng.zhou@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22436 3bd1e089-480b-0410-8dfb-8563597acbee