fix: Empty name and/or email are valid use cases.

[k0ral]

As a matter of fact, git does not fully enforce that name and/or email are non-empty. I don't know how to get there, but as an example, from hantsy/symfony-rest-sample:

commit 6639b2d5316ad1e41a3264199f7a91ba9b13a32d
Author:  <>
Date:   Sun Feb 13 09:17:25 2022 +0000

    Deployed 57b0a9f with MkDocs version: 1.2.3

Thus, I believe go-gitdiff should support this use-case.

[zricethezav]

@k0ral great addition, thanks!