chore: Skip transaction when no preview exists

[SystemKeeper]

Random finding while looking for something else. No need to start a transaction if we are not doing anything anyway.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[SystemKeeper]

/backport to stable35

[CarlSchwan]

/backport to stable34

[CarlSchwan]

/backport to stable33

[backportbot]

The backport to stable34 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable34
git pull origin stable34

# Create the new backport branch
git checkout -b backport/65436/stable34

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 08f14193

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/65436/stable34

Error: Failed to create pull request: Validation Failed: {"resource":"PullRequest","code":"custom","message":"A pull request already exists for nextcloud:backport/65436/stable34."} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.