fix: ensure HMR doesn't mess with anchor nodes

[dummdidumm]

The refactoring in #12215 didn't take HMR into account. As a result, the anchor was passed to the HMR block, which was subsequently cleaned up on destroy - but the anchor could be shared with other components and therefore needs to stay in the dom. Passing null instead solves the problem.
Fixes #12228
Fixes #12230
Fixes #12233

The test simulated what SvelteKit does, where this bug manifests the most prominent on page changes.

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