fix: Handle None for replace field in ReplaceColOp schema

[sentry]

This PR addresses issue DALGO-BACKEND-2B3, where a ValueError occurred during the validation of 'replace' operations. The root cause was a mismatch between the frontend sending None for the replace field (to signify replacing with an empty string) and the backend's Pydantic schema ReplaceColOp expecting a non-nullable str.

The fix updates the ReplaceColOp schema in ddpui/schemas/dbt_workflow_schema.py to make the replace field Optional[str] with a default value of an empty string (""). This allows None values from the frontend to be correctly processed, preventing the validation error.

Fixes DALGO-BACKEND-2B3

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.66%. Comparing base (75cb6fd) to head (27a1bea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1379   +/-   ##
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  Coverage   58.66%   58.66%           
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  Files         132      132           
  Lines       15690    15690           
=======================================
  Hits         9205     9205           
  Misses       6485     6485           

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