Fix JS error when discussion has no diff_file

Merged Phil Hughes requested to merge ph/230900/fixGetDicussionJSError into master

What does this MR do?

Fixes a JS error that would get thrown when a discussion has no diff_file attached to it in merge requests. This can happen when the user opens up a merge request that is linked to a normal comment, not a diff comment.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

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

Closes #230900 (closed)