COMP: Fix "undefined-bool-conversion" clang warnings in vtkMRMLScene

Slicer/Libs/MRML/Core/vtkMRMLScene.cxx:533:9: warning: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Wundefined-bool-conversion]
if ( !this )
~^~~~
Slicer/Libs/MRML/Core/vtkMRMLScene.cxx:1492:9: warning: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Wundefined-bool-conversion]
if ( !this || !this->Nodes)
~^~~~

From: Hans Johnson <hans-johnson@uiowa.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24617 3bd1e089-480b-0410-8dfb-8563597acbee