livecheck: refactor HEAD-only formula handling

[samford]

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The existing code for handling a HEAD-only formula involves two return values that can be nil but this isn't apparent because the related methods aren't typed. This adds type signatures to the methods and updates the livecheck code to account for nil return values (making it clear which methods can return nil). This addresses T.must usage in the same code in #19323.

Besides that, this also extracts the "Unable to get versions" error message into a constant, like NO_CURRENT_VERSION_MSG.

[p-linnane]

Makes sense. Thanks!

[MikeMcQuaid]

Looks good, thanks! Some suggested style tweaks but feel free to merge with/without without a rereview.

[samford]

The suggested changes make sense to me (and pass brew style, brew typecheck, etc.), so I've integrated them in the latest push. I didn't see any issues when testing this locally (with brew livecheck), so I'm going to go ahead and merge. Thanks!