NIFI-15711 Get raw commit author name if commit not tied to any GH user

[Kamilkime]

Summary

NIFI-15711

When a commit author is not tied to any GitHub account - NPE is thrown. Now the raw author commit will be returned instead, and this author field is always present in a git commit.

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  • JDK 21
  • JDK 25

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[pvillard31]

Thanks @Kamilkime - I had added this fix as part of my PR #10972 but it's better to get this merged first and I'll then rebase my PR.