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

editor/editor_vcs_interface.cpp

@@ -30,7 +30,7 @@
 
 #include "editor_vcs_interface.h"
 
-EditorVCSInterface *EditorVCSInterface::singleton = NULL;
+EditorVCSInterface *EditorVCSInterface::singleton = nullptr;
 
 void EditorVCSInterface::_bind_methods() {
 	// Proxy end points that act as fallbacks to unavailability of a function in the VCS addon