Fix undefined method `head_commit_sha' for nil:NilClass

What does this MR do?

It seems merge_request_diff association is not found even though a merge request has been persisted. Because of that, we see that diff_head_sha raises an exception thus ended up in hammering sidekiq jobs.

I suspect this could happen when postgres replication lag is unstable. We should check the persistency of merge_request_diff in the method to properly fall back to gitaly call in such edge cases.

This MR is fixing https://sentry.gitlab.net/gitlab/gitlabcom/issues/1037137/?query=head_commit_sha.

close #35210 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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