feat: add no value option to edit string value ux-editor

[wrt95]

Description

• Added an option for when no value is set. The user can just select that value to remove their selection.

Related Issue(s)

• Not possible to remove currency formatting from "Kort svar" #13166

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.27%. Comparing base (2f17f0a) to head (e57182f).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13807   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files        1635     1635           
  Lines       21828    21836    +8     
  Branches     2567     2568    +1     
=======================================
+ Hits        20796    20804    +8     
  Misses        787      787           
  Partials      245      245           

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[Jondyr]

Neat 👍

[lassopicasso]

Tested in dev - works as expected 🥳