Fix edge case that makes GitHubImport UserFinder to fail

What does this MR do and why?

Fix an edge case that could cause the error:

Sentry Error link - internal only

undefined method `headers' for nil:NilClass

  etag = client.octokit.last_response.headers[:etag]

On !140051 (merged), UserFinder was updated to only allow one request at a time among all GitHub Import jobs. However, a bug was introduced, which caused the error above.

This change fixes the bug

Related to: #435340 (closed)

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