fix(ts): update `AppRouteHandlerFn` type to handle `null` response from middleware handler

[ajstars1]

☕️ Reasoning

In NextAuth Function in Middleware type null does not contain in overload

Before code updation 👇

Type Error: No overload matches this call.

Allow Null Return Value: The AppRouteHandlerFnContext type now includes null as a valid return value. This addresses the error: No overload matches this call : " Type 'null' is not assignable to type 'void | Response | Promise<void | Response>'"

After code updation 👇

🧢 Checklist

• [ ] Documentation
• [x] Tests
• [x] Ready to be merged

🎫 Affected issues

Fixed: #10047

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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[ndom91]

Hmm interesting.

@balazsorban44 it looks like Next.js itself just types this as unknown actually. Should we switch to that? Save ourselves some potential future trouble?

See: https://github.com/vercel/next.js/blob/canary/packages/next/src/server/future/route-modules/app-route/module.ts#L81-L91

PR which updated it last with some comments about why, etc.: https://github.com/vercel/next.js/commit/f47c40917485bac359df0e3c57d5394a7bcb1e90#diff-66b976b1650819b48a11d6d16d984057aa95192edcd52e936fbeec386fc9c8eb

[aifirstd3v]

still under review?