handling situation where user login is undefined

If GitHub API search is not able to resolve a committer to a valid GitHub user, the committer name is now listed without a link.

Before this fix, when the user login was resolved to <undefined> entries like this were produced:

- [bkimminich](https://github.com/bkimminich)
- [Horst Schlaemmer](https://github.com/undefined) aka `undefined`

As there actually is a user with that handle, he might unduly receive credit for commits he never made... ;-)

Example output after fix:

- [bkimminich](https://github.com/bkimminich)
- Horst Schlaemmer

Test case: Running `authors` on https://github.com/bkimminich/AgileSoftwareDevelopmentInPractice