Fix LambdaQueue.get_jobs method

[Marishka17]

Motivation and context

Since rq Job.fetch_many can return None we must explicitly check that job exists.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• 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 #7231 (69338e1) into develop (a9c3425) will increase coverage by 0.04%.
Report is 1 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7231      +/-   ##
===========================================
+ Coverage    81.50%   81.54%   +0.04%     
===========================================
  Files          365      365              
  Lines        39261    39261              
  Branches      3631     3631              
===========================================
+ Hits         31999    32015      +16     
+ Misses        7262     7246      -16     

Components	Coverage Δ
cvat-ui	75.41% <ø> (+0.08%)	⬆️
cvat-server	87.09% <100.00%> (ø)