[typescript-fetch] fix API signature return type for empty 20x responses

[danowensdev]

Fixes #20248

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[danowensdev]

As per the PR checklist, I am mentioning the typescript technical committee, as the only part changed here is the apis.mustache used by the typescript-fetch generator. @TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka