fix: root header doesn't display when there are no elements

[lassopicasso]

Description

Removed the logic that returned null when schemaModel.isEmpty() was true, as it blocked rendering the root header when only the root element was present. This check, along with the associated method and tests, has been removed.

Fixed a race condition that occurred after this removal by adding setQueryData for user info, ensuring the cache is ready since user info is fetched at a higher level in PageLayout.tsx.

Related Issue(s)

• Data model - Root header disappears when all elements are deleted #13388

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 93.74%. Comparing base (957f1cb) to head (ac85f5d).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13439      +/-   ##
==========================================
- Coverage   93.74%   93.74%   -0.01%     
==========================================
  Files        1446     1446              
  Lines       20030    20028       -2     
  Branches     2429     2428       -1     
==========================================
- Hits        18778    18776       -2     
  Misses       1004     1004              
  Partials      248      248              

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[JamalAlabdullah]

Looks good and it works as expected😊

[JamalAlabdullah]

Looks good 😊

[mlqn]

Tested OK in dev! 💯 I just noticed that a generic error is displayed when we try to generate the model, but this will be fixed by this issue: #11037

error.mov