fix: prevent empty strings from overwriting step content in data layer

[Pranjalya]

fixes #2789

Description
In the ChainlitDataLayer, the create_step method uses an INSERT ... ON CONFLICT DO UPDATE pattern with COALESCE. Because Step objects initialize input and output as "" (empty string) instead of None, the database treats the empty string as a valid value to update, rather than a "missing" value to ignore.

This PR ensures that input and output are passed as None (SQL NULL) if they are empty strings. This allows the existing COALESCE logic in the SQL query to correctly preserve existing content if a late-arriving "initial" save task executes after a "final" update task.

Changes
Modified backend/chainlit/data/chainlit_data_layer.py:

• Updated the params dictionary in create_step to convert empty input and output strings to None.

Testing
Verified using the reproduction script provided in the associated issue. With this change, the COALESCE function correctly identifies the NULL value and maintains the existing content even if the tasks finish out of order.

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Summary by cubic

Prevents empty input/output strings from overwriting existing step content by treating them as NULL in the data layer, preserving final step data when saves finish out of order.

• Bug Fixes
  • In ChainlitDataLayer.create_step, pass input/output as None when empty so SQL COALESCE keeps existing values; minor Ruff lint cleanup.

^{Written for commit 1d92838. Summary will update on new commits.}

[cubic-dev-ai]

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