support for null in ui4t operations

[Ishankoradia]

adds support for parsing null string to null object. tested operations

• generic column op
• case when op
• filter op
• replace op

Correspondingdbt-automation PR - DalgoT4D/dbt-automation#119

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 44.73%. Comparing base (12dc061) to head (0081ef5).

Files	Patch %	Lines
...Components/OperationPanel/Forms/CaseWhenOpForm.tsx	16.66%	5 Missing ⚠️
...nents/OperationPanel/Forms/GenericColumnOpForm.tsx	25.00%	3 Missing ⚠️
...ponents/OperationPanel/Forms/WhereFilterOpForm.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #875   +/-   ##
=======================================
  Coverage   44.73%   44.73%           
=======================================
  Files          77       77           
  Lines        5314     5322    +8     
  Branches     1232     1236    +4     
=======================================
+ Hits         2377     2381    +4     
- Misses       2803     2807    +4     
  Partials      134      134           

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