Fixed error when getting data from CS and cache=True

[azhavoro]

Motivation and context

Fix

AttributeError: 'tuple' object has no attribute 'tobytes'

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into the CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see 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.

[codecov]

Codecov Report

Merging #6932 (2c2f4f8) into develop (92906a7) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6932      +/-   ##
===========================================
+ Coverage    82.53%   82.60%   +0.06%     
===========================================
  Files          360      360              
  Lines        38895    38895              
  Branches      3544     3544              
===========================================
+ Hits         32101    32128      +27     
+ Misses        6794     6767      -27     

Components	Coverage Δ
cvat-ui	77.73% <ø> (+0.10%)	⬆️
cvat-server	87.04% <100.00%> (+0.03%)	⬆️

[nmanovic]

LGTM