Fix serialization of LegacyDiffNote

What does this MR do?

It checks if note's class is DiffNote (rather than using #diff_note? method which is true both for DiffNote and `LegacyDiffNote).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

LegacyDiffNote may have position set to empty string (instead of nil) which causes that .to_h raises an exception.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #49355 (closed)