presto-main/src/main/java/io/prestosql/operator/index/PageRecordSet.java/PageRecordCursor/isNull
class PageRecordCursor:        @Override
        public boolean isNull(int field)
        {
            checkState(position >= 0, "Not yet advanced");
            checkState(position < page.getPositionCount(), "Already finished");
            return page.getBlock(field).isNull(position);
        }