fix: don't assume boundary exists during increment/decrement (#18289)

Follow-up to #18273 (not merged yet hence no changeset here): We can run
into a null-pointer when wanting to increment/decrement inside an effect
root that is outside a the component tree. Similarly, if not using the
component logic to unset context at the right time we gotta do it
"manually" inside `save`.

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Co-authored-by: Rich Harris <rich.harris@vercel.com>
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>