Fix early constant folding for isNull/isNotNul and analyzer.

[KochetovNicolai]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix removing the WHERE and PREWHERE expressions, which are always true (for the new analyzer)

Fixes #64575

[robot-ch-test-poll4]

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[novikd]

Strange formatting

[novikd]

That's an interesting change. Is it because of distributed execution?

[KochetovNicolai]

No. Looks like variant just always can be nullable, this is implemented in selector.

[novikd]

Why can it be "empty"?

[KochetovNicolai]

We are returning std::nullopt if where condition is constant 1.