Allow empty label on Ultralytics YOLO dataset

[imyhxy]

Summary

In Ultralytics YOLO dataset, there is no label required for the background images. But in datumaro, the _check_ann_file_impl of the _YoloLooseImporter class will filter all empty label .txt and does not search for images without label .txt. This PR will not search for images without label .txt but will add images with empty label .txt to the dataset.

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

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (784e039) 80.54% compared to head (5b8b469) 80.54%.
Report is 1 commits behind head on develop.

Files	Patch %	Lines
src/datumaro/plugins/data_formats/yolo/importer.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1264      +/-   ##
===========================================
- Coverage    80.54%   80.54%   -0.01%     
===========================================
  Files          271      271              
  Lines        30438    30457      +19     
  Branches      5930     5932       +2     
===========================================
+ Hits         24517    24532      +15     
- Misses        4532     4536       +4     
  Partials      1389     1389              

Flag	Coverage Δ
ubuntu-20.04_Python-3.8	80.52% <78.94%> (-0.01%)	⬇️
windows-2022_Python-3.8	80.51% <78.94%> (-0.01%)	⬇️

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[vinnamkim]

LGTM. Thanks for fixing what we missed.