fix(encryption): do not setup filesystem without permissions

[susnux]

Summary

• resolves [Bug]: CalDAV/CardDAV sync fails with app password when file access is disabled – results in HTTP 503 #51898

If the current request does not have permissions for the filesystem we must not try to setup the filesystem.

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)

[susnux]

/backport to stable32

[susnux]

/backport to stable31

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31

# Create the new backport branch
git checkout -b backport/55236/stable31

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 7cc3c1f6

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/55236/stable31

Error: Failed to push branch backport/55236/stable31: remote: {"auth_status":"auth_error","body":"Invalid username or token. Password authentication is not supported for Git operations."}
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

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

[lxp]

Thank you for taking the time to fix this issue!
I manually applied the fix to Nextcloud 31.0.9 and it works fine.
There seems to be a problem with the backportbot. Should I manually open a PR for backporting?