Handle contributions with missing authors in data generation tool

[rmfranken]

The current tool may produce records of contributions to a repository from extinct Github users (authors who no longer have a profile), resulting in missing author triples (i.e. contributions are found but no pulse:contributor property is present). This creates incomplete or ambiguous data regarding repository history.

Task

• Update the data extraction/generation logic to handle cases where a contribution object is missing an author (the user no longer exists on Github).
• Determine the preferred solution: create placeholder authors.
• Implement the logic fix (do NOT manually patch the data) so future outputs/data have well-defined contributor/author statements, even when the profile is gone.
• Add tests to ensure correct handling of authorless contributions going forward.

Context/example:
See parent issue: Contributions without authors

E.g. the following record:

<https://github.com/RossComputerGuy_https://github.com/PCRE2Project/pcre2> a pulse:Contribution ;
    pulse:contributionCount 2 ;
    pulse:contributionTo <https://github.com/PCRE2Project/pcre2> ;
    pulse:firstContributionDate "2024-02-18T16:27:35+00:00"^^xsd:dateTime ;
    pulse:lastContributionDate "2024-03-15T16:59:58+00:00"^^xsd:dateTime .

Should not be missing the author or remain unlinked to a contributor entity.