presto-main/src/main/java/io/prestosql/operator/SimplePagesHashStrategy.java/SimplePagesHashStrategy/isChannelPositionNull
class SimplePagesHashStrategy:    private boolean isChannelPositionNull(int channelIndex, int blockIndex, int blockPosition)
    {
        List<Block> channel = channels.get(channelIndex);
        Block block = channel.get(blockIndex);
        return block.isNull(blockPosition);
    }