bump-*-pr: handle HOMEBREW_NO_GITHUB_API=1 being set

We early return here https://github.com/homebrew/brew/blob/92a4311868322188478d7a90511ec0e8e6b0d7df/Library/Homebrew/utils/github/api.rb#L220 , but don't then handle that through the stack.

Repro:

```console
❯ HOMEBREW_NO_AUTO_UPDATE=1 HOMEBREW_NO_GITHUB_API=1 brew bump-formula-pr --write-only --tag 1.2.3 --no-audit jq
Error: undefined method `[]' for nil:NilClass
Do not report this issue until you've run `brew update` and tried again.
Warning: Removed Sorbet lines from backtrace!
/opt/homebrew/Library/Homebrew/utils/github.rb:565:in `block in fetch_pull_requests'
/opt/homebrew/Library/Homebrew/utils/github/api.rb:334:in `paginate_graphql'
/opt/homebrew/Library/Homebrew/utils/github.rb:564:in `fetch_pull_requests'
/opt/homebrew/Library/Homebrew/utils/github.rb:628:in `check_for_duplicate_pull_requests'
/opt/homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:456:in `check_open_pull_requests'
/opt/homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:135:in `run'
/opt/homebrew/Library/Homebrew/brew.rb:89:in `<main>'
Rerun with `--verbose` to see the original backtrace
```

Library/Homebrew/utils/github.rb

@@ -562,6 +562,7 @@ def self.fetch_pull_requests(name, tap_remote_repo, state: nil, version: nil)
 
     pull_requests = []
     API.paginate_graphql(graphql_query, variables:) do |result|
+      return [] if result.nil?
       data = result["search"]
       pull_requests.concat(data["nodes"].select { |pr| regex.match?(pr["title"]) })
       data["pageInfo"]