Fix GitHub importer to import PR where source repo/fork was renamed/deleted

What does this MR do?

Fix "unknown repository" of a GitHub Pull Request after renamed or deleted the repo/fork. The GitHub Pull Requests API returns null for repo attribute of the source branch for these PRs.

Why was this MR needed?

Without this fix, GitHub Pull Requests where the source repo/fork was renamed or deleted won't be imported.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24594 (closed)