docs: keep dehydrated serializable on flight requests in the Next pages example

[dangreen]

Why

src/pages/characters.tsx in the Next pages example skips dehydration on flight requests:

let dehydrated

if (!isFlight(context.req.headers)) {
  dehydrated = await dehydrate(...)
}

return { props: { dehydrated } }

Next rejects an explicitly undefined prop returned from getServerSideProps in development (isSerializableProps, enabled under the dev server and build-time SSG). Reproduced with NANO_KIT_DEV=1 next dev --webpack and a real click on the CHARACTERS link from /episodes:

GET /_next/data/development/characters.json -> 500
GET /characters                             -> 200   (Next falls back to a full reload)

⨯ Error: Error serializing `.dehydrated` returned from `getServerSideProps` in "/characters".
Reason: `undefined` cannot be serialized as JSON. Please use `null` or omit this value.

Production is unaffected, since JSON.stringify drops the key, but the example demonstrates exactly the scenario that breaks in next dev.

What

The page is back to the single expression it had before 33b6e0b, which only switched to let because the navigation setup took two statements at the time:

const dehydrated = !isFlight(context.req.headers) && await dehydrate(
  Stores$,
  [
    provide(LocationNavigation$, virtualNavigation(context.resolvedUrl, routes))
  ]
)

A flight request now returns false, which Next serializes and HydrationProvider treats as "skip hydration".

Checks

• Same click after the change: _next/data/development/characters.json answers 200, no document request follows, the console is clean.
• oxlint on the file passes. The example's tsc reports only pre-existing noise from generated .next types and import.meta.env in the symlinked package sources.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (8794697) to head (9d67864).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #251      +/-   ##
==========================================
+ Coverage   83.29%   83.33%   +0.03%     
==========================================
  Files          98       98              
  Lines        2556     2556              
  Branches      551      551              
==========================================
+ Hits         2129     2130       +1     
  Misses        314      314              
+ Partials      113      112       -1     

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