fix: check for wrong return values from form actions

[dummdidumm]

Related to #8523

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[benmccann]

Related to https://github.com/sveltejs/kit/issues/8523

Should this close that issue or is there more to do?

[dummdidumm]

It's related because the last comment in there comes from wrong usage, but the messages before that sound mysterious, so I'd like to keep it open at least until Rich can have a look - he probably knows more about what changed in devalue recently that could have caused this.

[Rich-Harris]

I think validation should only happen in dev, so I amended the PR (I also think we should strive to keep side-effects and return statements separate, as it makes code easier to follow).

However, doing so revealed an issue with the types — we previously had this...

data: /** @type {Record<string, any> | undefined} */ (data)

...and removing the annotation caused an error...

Type 'void | Record<string, any>' is not assignable to type 'Record<string, any> | undefined'

...that I think is best fixed by changing the interface from data?: Success to data: Success | void which more accurately reflects the ground truth. But I'll hold off merging for the moment in case that's the wrong answer