[git extension] fix existing user_override_name key, but with None value

[muritane]

When trying to git commit found out that the ~/.gitconfig file was mounted to /home/None/.gitconfig.
The corresponding test is also added.

[tfoote]

Reproduced with rocker --git --user -- ros:rolling

The get method is only testing for defined, but now that the value has a default value it is defined but still None.

Thanks for the fix with a test to confirm coverage.