fix(attachments): Don't require document session for getting attachments

[mejo-]

In editors with a user or share token we don't want to depend on a document session for fetching attachments.

This fixes fetching attachments in editors without a document session but with a user session or share token, e.g. in view mode of the Collectives app.

Fixes: nextcloud/collectives#1201

🏁 Checklist

• [x] Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• [x] Sign-off message is added to all commits

[mejo-]

/backport to stable29

[mejo-]

/backport to stable28