fix: ActionData type - disallow undefined, allow null

[tcc-sejohnson]

Closes #7961. Prior art: #6994.

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Latest commit: 3351698

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@sveltejs/kit	Patch

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[dummdidumm]

Thanks for pointing out the inconsistency about undefined/null - there was indeed one situation where undefined could sneak in, where it shouldn't.

[dummdidumm]

almost forgot, this also needs a changeset