Fixes cloud storage

[Marishka17]

Motivation and context

• exception with getting content when specific_attributes='' (detail)
• no deletion of temporary index for temporary manifest when getting content
• fixed getting owner structure instead of id
• deleted :=
• fixed creating task with cloud storage: preview && nonexistent field

Resolved #3340

How has this been tested?

Manually

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