fix: preserve index field in updateBatch() when updateOnlyChanged is true

[michalsn]

Description
This PR fixes a bug where updateBatch() threw an InvalidArgumentException when updateOnlyChanged was true (the default). The index field was being stripped as "unchanged" by transformDataToArray(), even though it's required for the batch WHERE clause.

Fixes #9943

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide