fix: handle missing fork repo

[xiantang]

This pull request improves the handling of pull requests where the head repository is missing or null, ensuring the application does not incorrectly treat such PRs as forks. It also adds a targeted test case for this scenario and refines the utility functions to be more robust against missing repository data.

Test improvements:

• Added a test in handle-pull-request.test.ts to verify that pull requests without a head repository are not incorrectly identified as forks.

Bug fixes and robustness:

• Updated the isFork utility in utils.ts to safely handle cases where head or repo may be null or undefined, preventing runtime errors.
• Enhanced the generatePullRequestWebhook test utility to support a headRepo parameter, allowing tests to simulate pull requests with missing or custom head repositories. [1] [2]

[xiantang]

@gr2m could u help take a look this issue?