Avoid 500's when serializing legacy diff notes

What does this MR do?

Legacy diff notes doesn't have persisted position's. Therefore we should special-case that.

This MR moves some logic to FileCollection::MergeRequestDiff and add additional tests.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54793

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer