presto-spi/src/main/java/io/prestosql/spi/block/AbstractMapBlock.java/AbstractMapBlock/isNull
class AbstractMapBlock:    @Override
    public boolean isNull(int position)
    {
        checkReadablePosition(position);
        boolean[] mapIsNull = getMapIsNull();
        return mapIsNull != null && mapIsNull[position + getOffsetBase()];
    }