Check if version has entity before trying to access it

[solracsf]

• Resolves: [Bug]: TypeError: OCP\AppFramework\Db\QBMapper::delete(): Argument #1 ($entity) must be of type OCP\AppFramework\Db\Entity, null given #37407 (comment)
• Completes PR Check if version entity is not null before delete #38125

Summary

Don't try to read a non-existent key (if version doesn't have an entity).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[solracsf]

/backport to stable26

[artonge]

CI failure unrelated