Style: Replaces uses of 0/NULL by nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

core/core_string_names.cpp

@@ -30,7 +30,7 @@
 
 #include "core_string_names.h"
 
-CoreStringNames *CoreStringNames::singleton = NULL;
+CoreStringNames *CoreStringNames::singleton = nullptr;
 
 CoreStringNames::CoreStringNames() :
 		_free(StaticCString::create("free")),