`brew bump-formula-pr` is encountering a type error, as the inferred
return type of `GitHub#check_for_duplicate_pull_requests` doesn't
align with the explicit return type of `#check_pull_requests`:
```
Error: Return value: Expected type T.nilable(T::Array[String]), got
type Module with value T::Private::Types::Void::VOID
Caller: /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/
bump-formula-pr.rb:137
Definition: /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/
dev-cmd/bump-formula-pr.rb:472 (Homebrew::DevCmd::BumpFormulaPr
#check_pull_requests)
```
This addresses the issue by adding a type signature with a `void`
return type to `#check_for_duplicate_pull_requests` and setting the
return type of `#check_pull_requests` to `void` as well. The return
type from `#check_pull_requests` isn't used, so a `void` return type
is arguably a better reflection of the method's behavior. The
`#check_pull_requests` method in `BumpCaskPr` has a `void` return
type, so this change brings the `BumpFormulaPr` method in line.
24ac6967ba