Fix: Cannot convert LabelMe dataset, that has no subsets

[yasakova-anastasia]

Summary

• Fixed the problem [Bug] Cannot convert LabelMe dataset, that has no subsets #289

How to test

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

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  Feel free to contact the maintainers if that's a concern.
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# Copyright (C) 2021 Intel Corporation
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# SPDX-License-Identifier: MIT