fix: Exclude null props in layout sets response

[lassopicasso]

Description

When fetching layout sets, we previously did not account for cases where the type or tasks properties were set to null. These properties should not be included in the JSON response when they are null. This issue has been resolved by adding the [UseSystemTextJson] annotation.

Before this fix, the following message would appear in the preview when the issue occurred:

And with the response:

Related Issue(s)

• #{issue number}

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 95.30%. Comparing base (5c1e97e) to head (c053d2d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13841   +/-   ##
=======================================
  Coverage   95.30%   95.30%           
=======================================
  Files        1652     1652           
  Lines       21980    21980           
  Branches     2585     2585           
=======================================
  Hits        20947    20947           
  Misses        787      787           
  Partials      246      246           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[JamalAlabdullah]

Tested ok in dev