COMP: Fix for conversion to non-pointer like type warning

Fixes the following warning (Wrapping):

ITK/Modules/Core/Common/include/itkCellInterface.h:368:61: warning: converting to non-pointer type 'itk::CellInterface<double, itk::CellTraitsInfo<2, double, double, long unsigned int, long unsigned int, long unsigned int, itk::Point<double, 2u>, itk::MapContainer<long unsigned int, itk::Point<double, 2u> >, std::set<long unsigned int> > >::CoordRepType {aka double}' from NULL [-Wconversion-null]
CoordRepType GetBoundingBoxDiagonalLength2(void) { return NULL; }

Change-Id: I88c79d7ebf73e38a611f773cd9242684fbbdec72
Merge topic 'FixWarningNULL'

f79b9f8d COMP: Fix for conversion to non-pointer like type warning