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

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

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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