Allow saving empty XMLs in VOC

[zhiltsov-max]

Motivation and context

Annotation-less XMLs were not saved, while they should.

How has this been tested?

Manual test

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

[nmanovic]

Here is an overview of what got changed by this pull request:

Issues
======
+ Solved 1
           

Complexity increasing per file
==============================
- datumaro/datumaro/plugins/voc_format/converter.py  2
         

See the complete overview on Codacy

[coveralls]

Pull Request Test Coverage Report for Build 6388

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 65.191%

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Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	2	77.06%
datumaro/datumaro/plugins/voc_format/converter.py	2	88.67%

Totals
Change from base Build 6382:	-0.02%
Covered Lines:	11059
Relevant Lines:	16556

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💛 - Coveralls