Fixed: return empty object instead of data:null in operation results

[bkoelman]

According to the spec at https://jsonapi.org/ext/atomic/:

An empty result object ({}) is acceptable for operations that are not required to return data.

This PR changes the response to align with the spec by returning an empty object instead of { "data": null }.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.81%. Comparing base (0941d7c) to head (81b82ad).

Files	Patch %	Lines
...ation/JsonConverters/WriteOnlyDocumentConverter.cs	72.72%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1564      +/-   ##
==========================================
- Coverage   90.83%   90.81%   -0.02%     
==========================================
  Files         348      348              
  Lines       11158    11168      +10     
  Branches     1828     1831       +3     
==========================================
+ Hits        10135    10142       +7     
- Misses        673      675       +2     
- Partials      350      351       +1     

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