Fix #1478 ack() is not accessible in global middleware in TypeScript

[seratch]

Summary

This pull request resolves #1478. The type of ack in app.use arguments had been AckFn | never. This pull request removes the pattern of never, so that the type is now AckFn | undefined.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1482 (7138b0b) into main (a48430e) will not change coverage.
The diff coverage is n/a.

❗ Current head 7138b0b differs from pull request most recent head 5f73338. Consider uploading reports for the commit 5f73338 to get more accurate results

@@           Coverage Diff           @@
##             main    #1482   +/-   ##
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  Coverage   81.99%   81.99%           
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  Files          18       18           
  Lines        1494     1494           
  Branches      435      435           
=======================================
  Hits         1225     1225           
  Misses        172      172           
  Partials       97       97           

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[srajiang]

Thanks for this @seratch! :)

[mwbrooks]

Small change, big impact! 👍🏻