presto-main/src/main/java/io/prestosql/sql/relational/SqlToRowExpressionTranslator.java/SqlToRowExpressionTranslator/visitNullLiteral
class SqlToRowExpressionTranslator:        @Override
        protected RowExpression visitNullLiteral(NullLiteral node, Void context)
        {
            return constantNull(UnknownType.UNKNOWN);
        }