[#70243] Make side-by-side diff frozen string compatible

[nsalguero]

Ticket

https://community.openproject.org/projects/openproject/work_packages/70243/github

What are you trying to accomplish?

Fix error 500 when displaying side-by-side diff in a repository.

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation in Lookbook (patterns, previews, etc)
• [X] Tested major browsers (Chrome, Firefox, Edge, ...)

[github-actions[bot]]

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[nsalguero]

I have read the CLA Document and I hereby sign the CLA

[myabc]

@nsalguero thank you for your contribution!

I believe #21504 tries to fix the same thing - however I prefer your solution.