Fix NullPointerException in EXTRA_READ validation

[brfrn169]

Description

This PR fixes a bug in the EXTRA_READ validation. In this bug, NullPointerException occurs during the EXTRA_READ validation when scanning records in a transaction, but some of them are deleted by other transactions.

Related issues and/or PRs

N/A

Changes made

• Changed Optional.of() to Optional.ofNullable() in Snapshot.toSerializableWithExtraRead() to avoid NullPointerException

Checklist

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes.
• Any remaining open issues linked to this PR are documented and up-to-date (Jira, GitHub, etc.).
• Tests (unit, integration, etc.) have been added for the changes.
• My changes generate no new warnings.
• Any dependent changes in other PRs have been merged and published.

Additional notes (optional)

N/A

Release notes

Fixed a bug where NullPointerException occurs during the EXTRA_READ validation when scanning records in a transaction, but some of them are deleted by other transactions.

[komamitsu]

LGTM! Thank you!

[jnmt]

LGTM! Thank you for fixing this. Sorry for keeping it on hold. I no longer have to hurry for my experiments, but let's discuss how we handle it in a better way when we get a chance.

[feeblefakie]

LGTM! Thank you!