COMP: qSlicerSubjectHierarchyPluginLogic: Fix -Wnull-conversion

This commit fixes the following warning:

/path/to/Slicer/Modules/Loadable/SubjectHierarchy/Widgets/qSlicerSubjectHierarchyPluginLogic.cxx:355:102: warning: implicit conversion of NULL constant to 'vtkIdType' (aka 'long long')
[-Wnull-conversion]
qSlicerSubjectHierarchyPluginHandler::instance()->pluginsForAddingNodeToSubjectHierarchy(node, NULL);

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git-svn-id: http://svn.slicer.org/Slicer4/trunk@25831 3bd1e089-480b-0410-8dfb-8563597acbee