COMP: fix nullptr_t issue in OSSIM 2.2.0 with clang

Modules/Adapters/OSSIMAdapters/src/otbDEMHandler.cxx

Modules/Adapters/OSSIMAdapters/src/otbRPCSolverAdapter.cxx

Modules/ThirdParty/Ossim/include/otb_ossim.h

...es/ThirdParty/OssimPlugins/include/ossimPluginConstants.h

...les/ThirdParty/OssimPlugins/src/ossim/ossimPluginCommon.h

...rdParty/OssimPlugins/src/ossim/ossimRadarSat2TiffReader.h

...s/ThirdParty/OssimPlugins/src/ossim/ossimSentinel1Model.h

...irdParty/OssimPlugins/src/ossim/ossimTerraSarProductDoc.h

...irdParty/OssimPlugins/src/ossim/ossimTerraSarTiffReader.h