presto-main/src/main/java/io/prestosql/sql/relational/SqlToRowExpressionTranslator.java/Visitor/visitNullIfExpression
class Visitor:        @Override
        protected RowExpression visitNullIfExpression(NullIfExpression node, Void context)
        {
            RowExpression first = process(node.getFirst(), context);
            RowExpression second = process(node.getSecond(), context);

            return new SpecialForm(
                    NULL_IF,
                    getType(node),
                    first,
                    second);
        }