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/rename_dialog.h

@@ -110,7 +110,7 @@ public:
 	void reset();
 	void rename();
 
-	RenameDialog(SceneTreeEditor *p_scene_tree_editor, UndoRedo *p_undo_redo = NULL);
+	RenameDialog(SceneTreeEditor *p_scene_tree_editor, UndoRedo *p_undo_redo = nullptr);
 	~RenameDialog(){};
 };