FIX: Handle empty data + tests

[bewithgaurav]

Work Item / Issue Reference

AB#<WORK_ITEM_ID>

GitHub Issue: #205

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Summary

This pull request improves the handling of empty string and binary values in the MSSQL Python bindings, ensuring that empty data is correctly distinguished from NULL values and does not cause assertion failures. It also adds comprehensive tests to verify correct behavior for these edge cases.

Improvements to empty value handling:

• Updated SQLGetData_wrap in ddbc_bindings.cpp to append an empty string ("") or empty bytes (b"") when the returned data length is zero, instead of causing assertion failures or misinterpreting the value as NULL. This change applies to both string and binary column types. [1] [2] [3]
• Changed assertions in FetchBatchData to allow zero-length data, ensuring that empty values are handled gracefully instead of triggering errors.

Testing improvements:

• Added new tests in tests/test_004_cursor.py to verify correct handling of empty strings and binary data, including distinguishing empty values from NULLs, batch fetching, and various edge cases for empty strings. These tests ensure that the fixes work as intended and prevent regressions.

Minor code cleanup:

• Minor formatting and comment improvements in ddbc_bindings.cpp for better code clarity.

[Copilot]

Pull Request Overview

This PR fixes empty data handling in the DDBC bindings by properly handling cases where SQL data has zero length. The change ensures that empty strings are correctly processed instead of being incorrectly handled by existing assertion logic.

• Added explicit handling for zero-length data in SQLGetData_wrap function
• Updated assertion to allow zero-length data in FetchBatchData function

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[sumitmsft]

@bewithgaurav Left a few comments. Please see if we can fix them