[BUG] IS NOT NULL renders as IS NULL (NOT dropped on round-trip)

[basili4-1982]

Bug Description

Round-trip rendering drops the NOT qualifier: a query WHERE email IS NOT NULL
is re-emitted as WHERE email IS NULL.

Root cause

pkg/sql/parser/expressions_operators.go stores IS NOT NULL as
BinaryExpression{Operator: "IS NULL", Not: true}. BinaryExpression.SQL()
(pkg/sql/ast/sql.go) renders the operator string verbatim and ignores the
Not flag in the null branch, silently inverting the predicate.

To Reproduce

tree, _ := gosqlx.Parse("SELECT * FROM users WHERE email IS NOT NULL")
fmt.Println(tree.SQL())
// SELECT * FROM users WHERE email IS NULL  (NOT lost!)

Expected Behavior

SELECT * FROM users WHERE email IS NOT NULL

Notes

Exists in v1.14.0 and current main (31b5b1c). Affects both PostgreSQL and
ClickHouse dialects. PR: #529.