Move prepare_nql_query to a separate function

[AleksanderWWW]

Before submitting checklist

• Did you update the CHANGELOG? (not for test updates, internal changes/refactors or CI/CD setup)
• Did you ask the docs owner to review all the user-facing changes?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e1e090e) 79.95% compared to head (145edb2) 76.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1568      +/-   ##
==========================================
- Coverage   79.95%   76.34%   -3.62%     
==========================================
  Files         286      287       +1     
  Lines       14360    14362       +2     
==========================================
- Hits        11481    10964     -517     
- Misses       2879     3398     +519     

Flag	Coverage Δ
e2e-management	?
e2e-s3	?
e2e-s3-gcs	?
e2e-standard	?
macos	73.31% <100.00%> (-6.30%)	⬇️
py3.7	73.60% <100.00%> (-5.77%)	⬇️
py3.9	?
ubuntu	73.46% <100.00%> (-6.16%)	⬇️
unit	73.60% <100.00%> (-0.57%)	⬇️
windows	72.77% <100.00%> (-6.85%)	⬇️

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