Backport to branch(3) : Fix handling of "null" values for non-TEXT columns in CSV imports

[feeblefakie]

This is an automated backport of the following:

• Original PR: Fix handling of "null" values for non-TEXT columns in CSV imports #3160
• Backported commit: 2cf54ec

Please merge this PR after all checks have passed.

[gemini-code-assist]

Summary of Changes

Hello @feeblefakie, 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 is an automated backport that addresses a critical issue in the data loader's CSV import functionality. It ensures that the string 'null' is properly converted into an actual null value when encountered in non-textual database columns, preventing incorrect literal string insertions. Concurrently, it explicitly preserves the literal 'null' string for text-based columns, aligning data interpretation with expected behavior and improving the robustness of data imports.

Highlights

• Improved Null Handling: The data loader now correctly interprets the string 'null' (case-insensitive) as an actual null value for all non-TEXT column types during CSV imports.
• Preservation of 'null' for TEXT columns: The change ensures that for TEXT column types, the string 'null' is still treated as a literal string and not converted to an actual null value, maintaining data integrity.
• Enhanced Test Coverage: New unit tests have been added to comprehensively verify the correct behavior of 'null' string handling across various data types, including both non-TEXT and TEXT columns.

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[gemini-code-assist]

Code Review

The pull request correctly implements the handling of string "null" values for non-TEXT columns by converting them to actual null values. The change in ColumnUtils is concise and correct. The accompanying tests are a good start, but I have a couple of suggestions to make them more comprehensive and maintainable by using parameterized tests, which will also help cover all relevant data types.