Blocks notes on transformed diff lines without mapping

What does this MR do and why?

Due to changes introduced on !75500 (merged), on side-by-side view we were showing the line number as "0" instead of empty text.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable rendered_diffs_viewer
2. Open any commit with a ipynb diff (a few can be found on gitlab-org/gitlab test, on branch add-ipython-files)
3. Switch to side-by-side
4. Make sure no line numbers appear as "0"
5. Make sure only lines with numbers can receive notes
6. Switch to raw diff, make sure only line numbers that appear on the raw diff have a number on the cleaner diff

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.