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

Merged Douglas Barbosa Alexandre requested to merge fix/github-importer into master

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?

What are the relevant issue numbers?

Closes #24594 (closed)

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