fix(vercel): fix handle() return type for Next.js 15 App router

[mddanishh18]

Fixes #4877

What this PR does:
Updates the return type of the Vercel handle() function to include void (and Promise<Response | void>). This is required because Next.js 15 App Router expects route handlers to optionally return void.

[yusukebe]

Hi @mddanishh18

Thank you for the PR. Does this mean there is a possibility app.fetch(req) returns void?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.89%. Comparing base (1aa32fb) to head (989b298).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4881   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files         177      177           
  Lines       11797    11797           
  Branches     3515     3514    -1     
=======================================
  Hits        10959    10959           
  Misses        837      837           
  Partials        1        1           

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