fix: error caused by empty commit

[XiongAmao]

If git has an empty commit, it will cause error in history.
I created a repository to reproduce this issue. https://github.com/XiongAmao/ob-git-plugin-issue

[Vinzent03]

Thanks for your fix, but I committed it now directly to the master branch (2571473), because I would use different syntax and proper formatting. Thanks for finding the issue though!