fix(sql): allow not() to accept SQL | undefined (and/or return type)

[Goldyvaiiii]

What

Fixes #1818

The not() function previously only accepted SQLWrapper, causing a TypeScript
error when composing it with and() or or(), which both return SQL | undefined:

// ❌ Before — TypeScript error
not(and(ilike(users.name, '%asdf%')))

// ✅ After — works perfectly
not(and(ilike(users.name, '%asdf%')))

[Goldyvaiiii]

Hi! I’ve submitted a fix for this.

Updated not() to support SQL | undefined, aligning it with and() and or(). This allows not(and(...)) and not(or(...)) without casting, and ensures not(undefined) returns undefined consistently.