Backport to branch(3) : Fix null condition handling

[feeblefakie]

This is an automated backport of the following:

• Original PR: Fix null condition handling #238
• Backported commit: 5cb38c1

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 integrates a critical fix related to null condition handling within the generic contracts' table scanning mechanism. It enhances the robustness of condition validation for primary and index keys, preventing the use of inappropriate operators and ensuring that queries are constructed correctly and safely. The changes are accompanied by new tests to validate the improved error handling.

Highlights

• Automated Backport: This pull request is an automated backport of a previous change, specifically commit 5cb38c1cd2070d9455765c599242717eff2e1bfc from original PR #238.
• Improved Condition Handling: The logic for validating primary and index key conditions has been refined in Scan.java. The methods isPrimaryKeyCondition and isIndexKeyCondition now explicitly check for the EQ operator early in their execution, ensuring that only equality conditions (or IS_NULL for index keys) are considered valid for these key types.
• New Test Cases: Two new test cases have been added to ScanTest.java to cover scenarios where invalid null conditions are applied to primary and index keys. These tests confirm that using IS_NULL for a primary key or IS_NOT_NULL for an index key correctly throws a ContractContextException.

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[gemini-code-assist]

Code Review

This backport correctly addresses a bug in null condition handling within the Scan contract. The modifications in isPrimaryKeyCondition and isIndexKeyCondition are logical and robust, preventing potential NullPointerExceptions by validating the operator before attempting to access a condition's value. The accompanying new tests are well-written, covering the specific edge cases for IS_NULL on primary keys and IS_NOT_NULL on index keys, ensuring the fix is effective. Overall, this is a good change that improves the contract's reliability.