Make CacheNodeSeedData match FlightRouterState more closely

[acdlite]

When PPR is enabled, the CacheNodeSeedData and FlightRouterState contained in an RSC payload (whether it's static or dynamic) should always have the same tree structure.

The only reason the two trees would ever be different is if the server partially renders with loading.tsx, but that behavior doesn't exist when PPR is enabled. (In that case, the CacheNodeSeedData would describe only a subset of FlightRouterState.)

The behavior is already correct but to make the types a bit closer, this changes the parallelRoutes slot of CacheNodeSeedData to be non-nullable; if there are no children, it should be an empty object.

It's not semantically important, I'm only changing it to make the types more consistent, since we typically traverse both trees in parallel.

Eventually we will probably merge these into a single transport type.

Closes NEXT-1868

[ijjk]

Tests Passed

[ijjk]

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	vercel/next.js canary	acdlite/next.js make-cachenodeseeddata-match	Change
buildDuration	12.8s	12.8s	N/A
buildDurationCached	7.2s	6.1s	N/A
nodeModulesSize	200 MB	200 MB	N/A
nextStartRea..uration (ms)	433ms	425ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js make-cachenodeseeddata-match	Change
170-HASH.js gzip	28.6 kB	28.7 kB	N/A
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	vercel/next.js canary	acdlite/next.js make-cachenodeseeddata-match	Change
_buildManifest.js gzip	484 B	482 B	N/A
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Rendered Page Sizes

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index.html gzip	529 B	526 B	N/A
link.html gzip	543 B	539 B	N/A
withRouter.html gzip	524 B	522 B	N/A
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Edge SSR bundle Size

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edge-ssr.js gzip	93.7 kB	93.7 kB	N/A
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Middleware size

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Next Runtimes

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app-page-exp...dev.js gzip	168 kB	168 kB	N/A
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app-route-ex...dev.js gzip	24 kB	24 kB	✓
app-route-ex..prod.js gzip	16.6 kB	16.6 kB	✓
app-route-tu..prod.js gzip	16.6 kB	16.6 kB	✓
app-route-tu..prod.js gzip	16.2 kB	16.2 kB	✓
app-route.ru...dev.js gzip	23.4 kB	23.4 kB	✓
app-route.ru..prod.js gzip	16.2 kB	16.2 kB	✓
pages-api-tu..prod.js gzip	9.37 kB	9.37 kB	✓
pages-api.ru...dev.js gzip	9.64 kB	9.64 kB	✓
pages-api.ru..prod.js gzip	9.37 kB	9.37 kB	✓
pages-turbo...prod.js gzip	21.9 kB	21.9 kB	✓
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pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	49.4 kB	49.4 kB	✓
Overall change	257 kB	257 kB	✓

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Commit: 1dc40f1