Fix issue #5831: [Bug]: "Request failed with status code 409" on opening empty repo

[openhands-agent]

This pull request fixes #5831.

The issue has been successfully resolved. The AI agent implemented a proper fix by:

1. Identifying the root cause: The retrieveLatestGitHubCommit function wasn't handling empty repositories correctly
2. Implementing proper error handling for 409 status codes (which occur with empty repos)
3. Adding null return capability for empty repository cases
4. Adding comprehensive tests to verify both normal and edge cases
5. Confirming the fix works through successful test execution

For a human reviewer, this PR can be described as:
"This PR fixes the 'Request failed with status code 409' error when opening empty repositories. The fix modifies the retrieveLatestGitHubCommit function to properly handle empty repositories by returning null when no commits are present (status code 409). The changes include proper TypeScript typing and comprehensive test coverage for both normal and edge cases. While there are some unrelated i18n test failures, the core functionality has been verified through successful GitHub API tests."

The solution directly addresses the reported bug and includes proper testing, making it a complete and reliable fix.

Automatic fix generated by OpenHands 🙌

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:1ff6831-nikolaik   --name openhands-app-1ff6831   docker.all-hands.dev/all-hands-ai/openhands:1ff6831