presto-main/src/main/java/io/prestosql/spi/expression/ConnectorExpressionTranslator.java/SqlToConnectorExpressionTranslator/visitNullLiteral
class SqlToConnectorExpressionTranslator:        @Override
        protected Optional<ConnectorExpression> visitNullLiteral(NullLiteral node, Void context)
        {
            return Optional.of(new Constant(null, typeOf(node)));
        }