Backport to branch(3.16) : 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 fix in the data loader's CSV import functionality. It resolves an issue where the string "null" was not consistently interpreted as a true null value for non-textual data types. The change ensures data integrity by properly converting "null" strings to actual nulls for appropriate column types, while explicitly maintaining the literal "null" string for text columns, thereby preventing potential data import errors and ensuring consistent data representation.

Highlights

• Improved CSV Import Handling: The data loader now correctly interprets the string "null" (case-insensitive) as a true null value for non-TEXT column types during CSV imports, preventing misinterpretation of data.
• Preserved TEXT Column Behavior: For TEXT columns, the string "null" will continue to be treated as a literal string "null", ensuring that actual text data is not inadvertently converted to a null value.
• Enhanced Test Coverage: New unit tests have been added to validate the correct handling of "null" strings across various data types, confirming the fix works as expected for both TEXT and non-TEXT columns.

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[gemini-code-assist]

Code Review

This pull request backports a fix to correctly handle the string "null" as a null value for non-TEXT columns during CSV imports. The change in ColumnUtils.java is concise and correctly implements this logic by checking for a case-insensitive "null" string and converting it to a null value before type parsing, except for TEXT columns.

The accompanying tests in ColumnUtilsTest.java verify the new behavior. I've added a couple of suggestions to refactor the new tests to be more concise and maintainable by using parameterized tests. I also noted that a few data types (BIGINT, FLOAT, BLOB) were missing from the test coverage for the "null" string handling, which would be good to add.