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/animation_track_editor_plugins.cpp

@@ -1263,7 +1263,7 @@ AnimationTrackEdit *AnimationTrackEditDefaultPlugin::create_value_track_edit(Obj
 		return memnew(AnimationTrackEditColor);
 	}
 
-	return NULL;
+	return nullptr;
 }
 
 AnimationTrackEdit *AnimationTrackEditDefaultPlugin::create_audio_track_edit() {