utils/pypi: fix an incorrect return type

[woodruffw]

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This return type is inconsistent with the parent signature, and was causing a checked type error with Sorbet on brew-pip-audit: https://github.com/Homebrew/brew-pip-audit/actions/runs/5654213254/job/15316828558

From a quick look, it appears to be vestigial from an older (pre-typechecked) version of this API. Returning nil instead makes sense, since the return here indicates the same error mode as the other nil returns above.

CC @alex

[woodruffw]

Proximate underlying cause here is konradhalas/dacite#235.

[MikeMcQuaid]

Thanks again @woodruffw!