presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/NullElementFinder/checkForNull
class NullElementFinder:        private void checkForNull()
        {
            nullElementPosition = -1;

            for (int i = 0; i < source.getPositionCount(); i++) {
                if (source.isNull(i)) {
                    nullElementPosition = i;
                    break;
                }
            }

            checkedForNull = true;
        }