fix(databricks-jdbc-driver): Return NULL decimal as NULL instead of 0

[paveltiunov]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: +0.02 🎉

Comparison is base (6cae9b5) 47.30% compared to head (3811908) 47.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6768      +/-   ##
==========================================
+ Coverage   47.30%   47.33%   +0.02%     
==========================================
  Files         155      155              
  Lines       20428    20430       +2     
  Branches     5285     5158     -127     
==========================================
+ Hits         9664     9670       +6     
- Misses       9923    10444     +521     
+ Partials      841      316     -525     

Flag	Coverage Δ
cube-backend	47.33% <50.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
...ubejs-schema-compiler/src/compiler/YamlCompiler.ts	71.25% <50.00%> (-0.27%)	⬇️

... and 42 files with indirect coverage changes

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