Fix reading, writing anno file and saving name of the subset for test subset

[yasakova-anastasia]

Summary

• Fixed saving the name of the test subset
• Changed: If there are no bounding boxes, only the image name is written to the file (previously, zero was written)

How to test

manually on the widerface dataset

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT