Import bitbucket issues that are reported by an anonymous user

What does this MR do?

When doing a bitbucket import, issues with an anonymous reporter where not imported. The reason is that the "reporter" field for such issues is "null" and this is incorrectly handled by the importer. This MR fixes the handling of "null" reporters and adds "Reported by Anonymous" to the issue description.

Are there points in the code the reviewer needs to double check?

This is my first contribution and I have limited experience with ruby, so please review carefully. Especially pay attention to the tests please, as I am not sure they are fully portable.

Why was this MR needed?

To fix import of anonymous bitbucket issues

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Fixes #44381 (closed)