[dir] fix clang-modernize warning, use nullptr in TDirectory.cxx
[dir] fix clang-modernize warning, use nullptr in TDirectory.cxx
olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,karies/root