public List<V> get(K key) {
        List<V> list = map.get(key);
        if (list == null) return List.of();;
        return list;
    }
Returns the List containing the elements with this key, or an empty list
if there are no elements for this key.
The returned list can be modified to add and remove values if the value exists.
vespajlib/src/main/java/com/yahoo/collections/ListMap.java