[FIX] Coerce NullLiteral to boolean in UnwrapCastInComparison

[gaurav8297]

Coerce NullLiteral to boolean type in UnwrapCastInComparison otherwise PredicatePushDown rule could throw an error while creating FilterNode with NullLiteral as a predicate. This is because FilterNode cannot be created with just NullLiteral without proper coercions.

Steps to reproduce the issue in the current version:

CREATE TABLE sample_table_1 (a smallint, b varchar);

CREATE TABLE sample_table_2 (a smallint, b varchar);

WITH t AS (SELECT a,
CASE 
  WHEN b = 'ABCD' THEN 'ABC'
  ELSE 'CDB' END AS channel 
  FROM sample_table_2 t2)
SELECT * FROM sample_table_1 t1 JOIN t ON t1.a = t.a WHERE channel = 'TESTING';

Error: Query 20230308_141912_00008_fy4xb failed: Predicate must be an expression of boolean type: null

Description

Additional context and related issues

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# Section
* Fix some things. ({issue}`issuenumber`)

[martint]

I found a much simple reproduction. @gaurav8297, would you mind putting up a PR to replace the existing test with this?

WITH t(a) AS (
    VALUES 'a', 'b'
)
SELECT *
FROM t t1 JOIN t t2 ON true
WHERE t1.a = 'aa';