Fixed saving of relative paths in dummy chunks instead of absolute

[Marishka17]

Motivation and context

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
  - [ ] I have updated the documentation accordingly
  - [ ] I have added tests to cover my changes
• I have linked related issues (read github docs)
  - [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage decreased (-0.02%) to 62.811% when pulling 397cc0a on mk/relative_paths_in_dummy_chunks into 5e2a9e3 on develop.

[nmanovic]

The change looks good and write. Tests work. Let's merge it.