fix: handling NULL values when convering mongo to sql

[sudipta9]

Description

Please include a summary of the changes and the related issue. Also include any relevant motivation and context.

Fixes # (issue)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

• Unit tests
• Integration tests
• Manual testing

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug where NULL values were not being correctly translated between MongoDB and SQL representations. The changes introduce comprehensive NULL handling across the conversion logic, ensuring that queries and data manipulations involving NULL are accurately reflected in the generated SQL and correctly interpreted when converting back to MongoDB. This significantly improves the reliability and correctness of the sql_mongo_converter module.

Highlights

• Enhanced NULL Handling in Mongo to SQL Conversion: Implemented robust handling for NULL values when converting MongoDB queries to SQL, specifically for direct equality, $eq, $ne, $in, and $nin operators. This ensures that IS NULL and IS NOT NULL clauses are correctly generated in SQL.
• Refactored Value Quoting: The quote_if_needed utility function was updated to explicitly return 'NULL' for None values, simplifying value formatting across various SQL generation functions like mongo_insert_to_sql and mongo_update_to_sql.
• Improved SQL to Mongo NULL Conversion: The convert_value function in the SQL to Mongo converter now correctly translates SQL 'NULL' literals (case-insensitive) and Python None inputs into Python None, ensuring accurate data type conversion.

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[gemini-code-assist]

Code Review

This pull request effectively addresses the handling of NULL values during Mongo to SQL conversion, which is a crucial fix for correctness. The changes for direct equality, $eq, $ne, and especially the complex logic for $in/$nin operators are well-implemented. The refactoring to use a centralized quote_if_needed function across mongo_insert_to_sql and mongo_update_to_sql is a great improvement for maintainability. The corresponding changes in sql_to_mongo.py to handle SQL NULL are also correct. I have one suggestion to refactor a section of the new code to reduce duplication and improve readability.