presto-main/src/main/java/io/prestosql/operator/SortedPositionLinks.java/SortedPositionLinks/isNull
class SortedPositionLinks:        private boolean isNull(int position)
        {
            long pageAddress = addresses.getLong(position);
            int blockIndex = decodeSliceIndex(pageAddress);
            int blockPosition = decodePosition(pageAddress);
            return pagesHashStrategy.isSortChannelPositionNull(blockIndex, blockPosition);
        }