Chore: Ensure Backward Sync Correctly Handles Finalized Block Hash

[pr9t]

PR description

This PR addresses a bug where backwards sync should correctly handle the finalized block hash 0x0000..0000.

Added an if condition in MergeCoordinator.updateFinalized() to:

• Check if the finalized block hash is 0x0000..0000.
• Log a warning instead of trying to import an invalid block.
• Return early to prevent unnecessary errors.

Fixed Issue(s)

fixes #7966

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• Considered documentation and added the doc-change-required label to this PR if updates are required.
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• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests