Remove default value checks

[ilongin]

Companion issue of https://github.com/iterative/studio/pull/11236
With making CH accept nullable columns, we no longer need special checks for default values in tests, as they will be None always.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.49%. Comparing base (13e5c13) to head (6f3f921).

Files with missing lines	Patch %	Lines
src/datachain/data_storage/db_engine.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
- Coverage   87.53%   87.49%   -0.05%     
==========================================
  Files         128      128              
  Lines       11369    11370       +1     
  Branches     1538     1538              
==========================================
- Hits         9952     9948       -4     
- Misses       1029     1033       +4     
- Partials      388      389       +1     

Flag	Coverage Δ
datachain	87.43% <66.66%> (-0.05%)	⬇️

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