presto-main/src/main/java/io/prestosql/type/setdigest/SetDigestStateSerializer.java/SetDigestStateSerializer/serialize
class SetDigestStateSerializer:    @Override
    public void serialize(SetDigestState state, BlockBuilder out)
    {
        if (state.getDigest() == null) {
            out.appendNull();
        }
        else {
            SET_DIGEST.writeSlice(out, state.getDigest().serialize());
        }
    }