GitHub's GraphQL API returns files: null (with no errors entry, and
changedFiles misreported as 0) when a PR's diff is too large to compute.
The unguarded pullRequest.files.nodes dereference in the fetcher crashed
the action with 'TypeError: null is not an object', and the formatter had
the same latent crash on prData.files.nodes.length.
Widen the GitHubPullRequest type to files | null so the compiler enforces
guards, degrade gracefully in the fetcher with a warning, and render the
file count as unavailable (not '0 files') in the formatter.
Fixes#1587
