Fix encoding detection for nil diffs

What does this MR do?

While migrating diffs to object storage on GitLab.com, I came across a curious issue affecting 6 (out of ~72M) diffs: an associated MergeRequestDiffFile where diff: nil: gitlab-com/gl-infra/production#1851 (closed)

I'm not sure what situation creates these, but it's easy enough to harden the migration process to permit them to be migrated. I remedied them manually on .com, but we should merge the fix for completeness.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

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

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