SSR Support: delete substate.error property (Next.js getServerSideProps)

[gfortaine]

Addresses points brought up in #88 (comment).

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[phryneas]

Hmm. I'm honestly flabbergasted by the stuff next.js is pulling there, but oh well.

I'd suggest we do delete substate.error here though - not unsetting it at all would lead to pretty weird behaviour in some situations.

[msutkowski]

How does everyone feel about just defaulting to null instead of undefined? Is that a better long-term solution for this?

[phryneas]

Well, it could also just not be present yet or have never been set. And I kinda don't see the point of setting a property to null when it has never been accessed yet - especially since we would have to do that to others then as well consequently.
So I'd rather just use delete X instead of X = undefined.

[gfortaine]

@phryneas PR updated with delete substate.error as suggested 👍

[msutkowski]

Thanks again for this @gfortaine! 🎉