Fixing coco keypoints export with absent points

[Eldies]

Motivation and context

Updating datumaro to fix bug cvat-ai/datumaro#114

• Tracks with no visible shapes will not be saved or loaded from the DB anymore
• Fixed export of skeletons with some points always hidden in COCO keypoints format

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.79%. Comparing base (5ce306e) to head (189057b).
Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9565      +/-   ##
===========================================
+ Coverage    73.77%   73.79%   +0.02%     
===========================================
  Files          448      448              
  Lines        46712    46772      +60     
  Branches      3943     3943              
===========================================
+ Hits         34462    34516      +54     
- Misses       12250    12256       +6     

Components	Coverage Δ
cvat-ui	77.74% <ø> (+<0.01%)	⬆️
cvat-server	70.73% <88.73%> (+0.04%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud