presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/UnnestBlockBuilder/appendNull
class UnnestBlockBuilder:    public void appendNull()
    {
        if (usingCopiedBlock) {
            outputBlockBuilder.appendNull();
        }
        else {
            int nullIndex = nullFinder.getNullElementIndex();
            if (nullIndex == -1) {
                startUsingCopiedBlock();
                appendNull();
            }
            else {
                appendId(nullIndex);
            }
        }
    }