Handle 404s when fetching GitHub user details

What does this MR do and why?

Handle 404s when fetching GitHub user details

The GitHub API can return a 404 when we try to fetch the user's details. We can see some errors when importing rails/rails. Specifically for the user Copilot.

We already have in place a fallback of using the GitHub username as the source user's name if we're unable to get the username from the API response.

This change does the same if we receive a 404 from the GitHub API.

MR acceptance checklist

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How to set up and validate locally

1. Enable github_user_mapping feature flag.
2. Conduct a GitHub import with user contributions from Copilot.
3. The source user should be created successfully with user name "Copilot"

Related to #508720 (closed)