Add guard for nil merge_request_diff.id

What does this MR do?

There's a somewhat obscure and rare case were @merge_request_diff can be a newly created and unsaved MergeRequestDiff object, which will exist for the purposes of the original guard, but will not have an id yet, and the code immediately following this guard relies on id being !nil?. Further testing on the frontend with @jboyson shows that even if we made it through this method without error, the frontend code has no idea what to do with a data blob that has an empty MergeRequestDiff object in it.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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
• [-] Security reports checked/validated by a reviewer from the AppSec team