chore: allow <svelte:element> effects to be pruned

[Rich-Harris]

'Inert' effects (those with no dependencies, or DOM, or teardown functions) do not get added to the effect tree, since there's no reason to keep them alive.

In two cases — {#await ...} and <svelte:element> effects — we return a noop function to prevent the effects from being pruned. In the case of await blocks this is necessary because a branch might not be created during the block's first run. In the case of <svelte:element> it's not — I think it was only there because we were previously updating the DOM manually rather than using the simpler assign_nodes mechanism, which organically prevents the effect being pruned.

In fact there's an edge case where we want the effect to be pruned — the case where this is non-reactive and falsy...

<svelte:element this={null} />

...and that's what happens with this PR.

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