Add Kleene nullability narrowing

[jongleb]

Description

This PR adds Kleene-logic nullability narrowing

CREATE TABLE t1 (pk INT NOT NULL, a INT NULL, b INT NULL);
CREATE TABLE t2 (d INT NULL);

A strict predicate is never true when its operand is NULL: NULL > 5 is not true, it is NULL. WHERE and ON keep only true rows. So the columns used in such a predicate are not null in the result, even if they are nullable in the DDL

SQL	before	after
WHERE a > 5	a: Int?	a: Int
WHERE a + 1 = 5	a: Int?	a: Int
WHERE a IN (1, 2)	a: Int?	a: Int
WHERE a = 1 AND b = 2	a, b: Int?	a, b: Int
JOIN t2 ON a = d	a, d: Int?	a, d: Int

No narrowing when NULL can still pass:

SQL	stays	why
WHERE a = 1 OR b = 2	a, b: Int?	other branch can be true
WHERE a <=> 5	a: Int?	null-safe comparison
LEFT JOIN t2 ON a = d	a, d: Int?	padded with NULL

Also fixed on the way: a window function is nullable when its frame can miss a row.

SQL	before	after
SELECT LAG(pk) OVER (ORDER BY pk)	Int	Int?