Modify coco to export annotations without media

[jihyeonyi]

Summary

Regarding Issue #1147 , modify coco exporter to be able to export annotations even if there's no media.
Annotations which need media information will be skipped as same as before.

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

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.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% 🎉

Comparison is base (2180b04) 79.98% compared to head (cb486f7) 80.01%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1158      +/-   ##
===========================================
+ Coverage    79.98%   80.01%   +0.02%     
===========================================
  Files          266      266              
  Lines        29967    29967              
  Branches      5901     5901              
===========================================
+ Hits         23970    23978       +8     
+ Misses        4637     4632       -5     
+ Partials      1360     1357       -3     

Flag	Coverage Δ
macos-11_Python-3.8	79.13% <100.00%> (+0.02%)	⬆️
ubuntu-20.04_Python-3.8	80.00% <100.00%> (+0.02%)	⬆️
windows-2019_Python-3.8	79.93% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/datumaro/plugins/data_formats/coco/exporter.py	88.96% <100.00%> (+1.05%)	⬆️

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[vinnamkim]

LGTM.