Adjust typing around #check_pull_requests

[samford]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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brew bump-formula-pr is encountering a type error (e.g., as seen in a recent homebrew/core autobump run), 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.

[MikeMcQuaid]

Thanks @samford!