infra: handle commits without associated PRs

[Ap4sh]

summary

• handle an empty response from GitHub's commit-to-PR endpoint in pr_helper
• keep the existing previous-commit fallback when an earlier commit is not tied to a PR
• add coverage for the helper response cases and the final main() fallback message

why

get_pull_request_number() already lets callers fall back to a previous commit link when a commit has no associated PR. GitHub can return 200 [] for that endpoint, but the helper currently indexes [0] first, which raises IndexError before the fallback runs

testing

• python infra/presubmit.py
• PYTHONPATH=infra pytest infra/pr_helper_test.py -q