presto-orc/src/main/java/io/prestosql/orc/reader/MapColumnReader.java/MapColumnReader/hasNull
class MapColumnReader:    private static boolean hasNull(Block keys)
    {
        for (int position = 0; position < keys.getPositionCount(); position++) {
            if (keys.isNull(position)) {
                return true;
            }
        }
        return false;
    }