nullableListSaver
fun <Original, Savable> nullableListSaver(saveNonNull: SaverScope.(Original) -> List<Savable>, restoreNonNull: (List<Savable>) -> Original?): Saver<Original?, Any>
listSaver for a nullable value.
null is represented by an empty list, so saveNonNull must return a non-empty list for every non-null value.