Do not crash if a commit has no author

[agateau-gg]

Context

There are some strange commits around there. This one for example, made ggshield crash because its Author field is mostly empty.

What has been done

Make commit parser accept empty authors, added unit-test.

Testing

• Unit-tests pass
• ggshield secret scan repo https://github.com/awslabs/amazon-eks-ami.git no longer fails

Needs #867 for CI to be green.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.81%. Comparing base (a16f080) to head (3b09986). Report is 1 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #866 +/- ## ======================================= Coverage 91.81% 91.81% ======================================= Files 170 170 Lines 7066 7066 ======================================= Hits 6488 6488 Misses 578 578 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/866/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/866/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.81% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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[agateau-gg]

Looks good, I just have a question: would it be possible for a commit to have no date, similarly to malformed commits with no author?

I hope not 😅 .

The way to produce a commit with no author is to actually set the git config value user.name to an empty string. The same thing can be done with user.email (which was already handled).

One can't do the same (AFAIK) for the date. It's possible to change the commit and author date with the $GIT_COMMITTER_DATE and $GIT_AUTHOR_DATE environment variables, so I just experimented messing with them: if they are empty git ignores them and uses the current date. If they contain invalid values it fails with a fatal: invalid date format message. We should be good 🤞🏻.