Don't create pr when there are no changes

[reddevilmidzy]

As is

  2025-11-13T23:44:10.495+09:00  INFO  Successfully checked out branch: queensac-1763045050471
    at src\git\repo.rs:181 on ThreadId(1)

  2025-11-13T23:44:10.495+09:00  INFO  No file changes to commit. Skipping commit creation.
    at src\git\pr_generator.rs:253 on ThreadId(1)

  2025-11-13T23:44:10.495+09:00  INFO  Pushing branch queensac-1763045050471 to remote origin
    at src\git\repo.rs:247 on ThreadId(1)

  2025-11-13T23:44:11.995+09:00  INFO  Successfully pushed branch queensac-1763045050471 to remote origin
    at src\git\repo.rs:267 on ThreadId(1)

  2025-11-13T23:44:11.996+09:00  INFO  Successfully pushed branch to remote
    at src\git\pr_generator.rs:300 on ThreadId(1)

  2025-11-13T23:44:12.607+09:00 ERROR  Failed to create PR: GitHub API error: Failed to create PR: GitHub
    at src\main.rs:87 on ThreadId(1)

To be

Do not create push & pr when there are no changes