Add comment and tests for zero address behavior in Ownable2Step.transferOwnership()

[PurrProof]

Fixes #5224

This pull request updates the Ownable2Step's contract transferOwnership method comments to clarify that using the zero address cancels a pending ownership transfer. Additionally, a new test case has been added to verify this behavior.

PR Checklist

• Tests
• Documentation
• Changeset entry (run npx changeset add)

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Latest commit: e79fbe4

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