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

No source files or tests are named. First reproduce the failure with the listed git config, repository, staging, and commit steps, then trace gitui's commit flow for handling an empty email. Done means a commit is created successfully with an empty configured email, with coverage for this scenario if the existing test structure supports it.

Written by the indexing model from the issue text.