Cannot commit with empty e-mail.

**Describe the bug**
When git is configured with an empty e-mail address, `gitui` does not allow commits.

**To Reproduce**
Steps to reproduce the behavior:
```
git config --global user.email "<>"
git init foo
cd foo
echo "some content" > a_file.txt
gitui
# Stage a_file.txt
# Hit "c" to start a commit
# Type any commit message and hit enter
```

**Expected behavior**
A new commit should be created.

**Screenshots**

**Context (please complete the following information):**
- Fedora 39
- GitUI 0.24.3
- Git 2.44.0

未指定源文件或测试。首先使用列出的 git config、仓库、暂存和提交步骤复现失败，然后跟踪 gitui 的提交流程以处理空电子邮件。完成的标准是：在配置的电子邮件为空的情况下成功创建提交；如果现有测试结构支持，则为此场景提供覆盖。

由索引模型根据 Issue 内容生成。