presto-main/src/main/java/io/prestosql/operator/PagesIndex.java/PagesIndex/isNull
class PagesIndex:    public boolean isNull(int channel, int position)
    {
        long pageAddress = valueAddresses.getLong(position);

        Block block = channels[channel].get(decodeSliceIndex(pageAddress));
        int blockPosition = decodePosition(pageAddress);
        return block.isNull(blockPosition);
    }