public List<Attribute> getOnAfterWindowData() {
        if (onAfterWindowData != null) {
            return onAfterWindowData;
        } else {
            return new ArrayList<Attribute>();  //return empty arraylist to avoid NPE
        }
    }
Class to hold meta info about StreamEvent.
This is also used to update variable positions on executors
modules/siddhi-core/src/main/java/io/siddhi/core/event/stream/MetaStreamEvent.java