Apply clang-tidy with modernize-use-nullptr,modernize-use-override

[chehrlic]

Since this project is using c++11, we should also use nullptr and override everywhere Converted with clang-tidy

• https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html
• https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-override.html