fix(migration): Check if table exists

[solracsf]

• Resolves: https://github.com/nextcloud/server/issues/48865

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
• [x] Backports requested where applicable (ex: critical bugfixes)

[solracsf]

/backport to stable30

[provokateurin]

Why would the table be missing? Is a previous migration already failing?