Resolve "[BUG] Investigate diff issues in production"

What does this MR do and why?

We are passing a nil actor on a Feature.enable(:jupyter_clean_diff) call, which causes bad behavior when we enabled it for specific projects.

Screenshots or screen recordings

How to set up and validate locally

1. Clone the repository: https://gitlab.com/gitlab-org/incubation-engineering/mlops/ipynb-test-projects/diff-sample into your local gdk installation

2. Enable the feature locally only for the created project

   Feature.enable(:jupyter_clean_diffs, my_project)

3. Open http://gdk.test:3000/root/diff_test/-/commit/542d42a0196e2e94a0d5793acdee5ba997ab12bf

4. On master, should crash with 'RangeError: 95..100 out of range'

5. Switching to the MR branch, should display the 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.

Related to #344676 (closed)