fix: handle patch-testing comment runs without PR context

[peter941221]

This fixes Patch Testing Cycles Comment runs that do not have a PR attached in the workflow_run payload.

The failure showed up in run 27173224829. GitHub returned pull_requests: [] for that run, but the workflow still tried to read .pull_requests[0].number and passed the result into post-to-github. The posting binary then unwrapped PR_NUMBER and panicked while decoding the GitHub comments response.

This change adds the guard in both places that assume PR context:

• the workflow now reads the first PR number with a null-safe jq expression and skips the posting step when no PR is attached
• post-to-github now treats a missing or blank PR_NUMBER as a no-op instead of panicking

The benchmark comparison path stays the same. If the run has a real PR, the workflow still posts the generated comparison comment through the existing code path.

Testing:

• cargo test -p sp1-test --bin post-to-github -- --nocapture
• cargo run --bin post-to-github with PR_NUMBER='' and empty cycle stats now exits 0 after printing the comparison table and a skip message