[fix] avoid unnecessary $page store updates

[dummdidumm]

Clicking a page link and staying on the same page with nothing changing still changed the $page.store, because

• form was always !== undefined (it's null)
• $page.error was set to undefined after hydration, but it should be null

No test because it's super silly to e2e test this - we really need client.js to be unit-testable.
Discovered while investigating another issue which is no longer reproducible.

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