Avoid Set in missing slots RSC payload

[hukuda222]

What?

Avoid sending the dev-only missingSlots value as a Set in the initial RSC payload. The payload now carries missing slots as a string array only when there are actually missing slots, and the client reconstructs the Set before providing MissingSlotContext.

Why?

In development, pages without missing parallel route slots can still include an empty Set at initialRSCPayload.m, which triggers this warning:

Only plain objects can be passed to Client Components from Server Components. Set objects are not supported.

This warning points users at application Server/Client Component props even though the Set comes from Next.js internals.

How?

• Serialize missingSlots as string[] | undefined in InitialRSCPayload.
• Omit m when the set is empty.
• Recreate Set<string> on the client only when missing slots are present.
• Add a regression assertion to the existing app dir RSC basics test.

Fixes #73323

Testing

• pnpm install
• pnpm build-all
• pnpm --filter=next types
• pnpm exec prettier --check packages/next/src/client/app-index.tsx packages/next/src/server/app-render/app-render.tsx packages/next/src/shared/lib/app-router-types.ts test/e2e/app-dir/rsc-basic/rsc-basic.test.ts
• pnpm lint-eslint packages/next/src/client/app-index.tsx packages/next/src/server/app-render/app-render.tsx packages/next/src/shared/lib/app-router-types.ts test/e2e/app-dir/rsc-basic/rsc-basic.test.ts
• pnpm test-dev-turbo test/e2e/app-dir/rsc-basic/rsc-basic.test.ts