Fix XML Encoding of Null Variant to conform to spec

[romanett]

Proposed changes

Don´t write <Null> Node if a null variant is encoded instead write <uax:Value xsi:nil=\"true\" />

Related Issues

• Fixes XML encoding of "null" Variant is incorrect #3056

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.05%. Comparing base (d5fd96b) to head (8880220).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3062      +/-   ##
==========================================
+ Coverage   57.02%   57.05%   +0.03%     
==========================================
  Files         357      357              
  Lines       68951    68956       +5     
  Branches    14156    14157       +1     
==========================================
+ Hits        39317    39343      +26     
+ Misses      25395    25371      -24     
- Partials     4239     4242       +3     

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[romanett]

This PR could be problematic considering #1771

[romanett]

This PR could be problematic considering #1771

-> mitigated by writing <uax:Value xsi:nil=\"true\" />

[mregen]

looks good..