Use nullptr instead of 0 or NULL (clang-tidy warns) (#603)

src/contrib/graphbuilder.cpp

src/contrib/graphbuilderadapter.cpp

src/contrib/graphbuilderadapter.h

src/node_data.cpp

src/nodebuilder.cpp

src/ostream_wrapper.cpp

src/scanner.cpp

src/scanner.h

src/simplekey.cpp