fix: Don't try to access null array

[solracsf]

Summary

{
  "reqId": "lG61fZdZOHdsj68Xjdto",
  "level": 3,
  "time": "2023-08-06T03:32:25+02:00",
  "remoteAddr": "",
  "user": "--",
  "app": "PHP",
  "method": "",
  "url": "--",
  "message": "Trying to access array offset on value of type null at /var/www/nextcloud/apps/federatedfilesharing/lib/Notifications.php#297",
  "userAgent": "--",
  "version": "26.0.4.2",
  "data": {
    "app": "PHP"
  },
  "id": "64d15c8ebc6e2"
}

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)

[kesselb]

Thank you 👍

I rebased the branch to have another ci run because drone mysql8.0-php8.0 failed.

[solracsf]

/backport to stable27

[solracsf]

/backport to stable26