private void addHandlerForAction(final Map<EntityAction, List<Handler>> map,
                                     final Handler handler,
                                     final EntityAction action) {
        map.computeIfAbsent(action, k -> new ArrayList<>()).add(handler);
    }
Check to see if we can find a handler for this event. If there isn't one
then there is no point in firing the event.
stroom-core/src/main/java/stroom/core/entity/event/EntityEventHandler.java