fix(back): Fix getting category name in COCO Importer

[magwyz]

The category_name field does not always exist in an annotation object.

[magwyz]

I got this issue from a dataset annotated with CVAT.

[cpvannier]

Thank you for the suggestion!

I left a few notes on what I would change, just a minor edit for readability.

Also, it would be great if you could format your code with Black so that it passes our lint test. You can install the VS Code extension and set it to format files automatically on save, or if you use another editor, you can run it directly with the PyPI package.

And finally, this update should be mentioned in the CHANGELOG.md file under Unreleased - Fixed:

- Fix COCO Importer to get the category name from the "categories" field as it does not always exist in the "annotations" field

Thanks again!