Writer field: fix content with non-breaking space

[distantnative]

Description

Summary of changes

Enforcing conversion of a non-breaking space to   as this is what ProseMirror internally will do automatically. And if the backend delivers the actual non-breaking space character but ProseMirror converts it to &nbsp: we have an unreasonable state where the current input value can never be the original value (and thus always shows as changed).

Changelog

Fixes

• Writer field: content with non-breaking changes doesn't anymore show up as changed always
  Writer: non-breaking spaces cause endless "Revert" state #6285

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion

[bastianallgeier]

Is there a good process to reproduce this issue? I'm trying to add non-breaking spaces, but I never really run into the issue myself on develop-minor. The fix looks good to me. I just really would love to see the difference IRL first :)

[distantnative]

@bastianallgeier this #6285 (comment) worked reliably for me. I added a text with a &nbsp: to the default template and copied it from there.

[bastianallgeier]

Thanks for pointing me to that comment. I was also able to reproduce it and the fix works great.