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`).
Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>
a7cacfff1c