support for `null` in ui4t operations

[Ishankoradia]

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

• [x] generic column op
• [x] case when op
• [x] filter op
• [x] replace op

Correspondingdbt-automation PR - https://github.com/DalgoT4D/dbt-automation/pull/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 :warning:
...nents/OperationPanel/Forms/GenericColumnOpForm.tsx	25.00%	3 Missing :warning:
...ponents/OperationPanel/Forms/WhereFilterOpForm.tsx	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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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