clang-modernize -use-nullptr

and s/\bNULL\b/nullptr/g for *.cpp/h/mm files not compiled on my machine

Source/Core/Core/Core.cpp

@@ -79,7 +79,7 @@ void EmuThread();
 bool g_bStopping = false;
 bool g_bHwInit = false;
 bool g_bStarted = false;
-void *g_pWindowHandle = NULL;
+void *g_pWindowHandle = nullptr;
 std::string g_stateFileName;
 std::thread g_EmuThread;