fix(UserTrait): Fix backend initialization

[provokateurin]

Summary

#47896 broke groupfolders tests with this error:

4) OCA\GroupFolders\Tests\Trash\TrashBackendTest::testHideDeletedTrashItemInDeletedParentFolderAcl
Error: Typed property OC\User\User::$backend must not be accessed before initialization

/home/jld3103/src/github.com/nextcloud/server/lib/private/User/User.php:104
/home/jld3103/src/github.com/nextcloud/server/apps/groupfolders/lib/ACL/UserMapping/UserMappingManager.php:27
/home/jld3103/src/github.com/nextcloud/server/apps/groupfolders/lib/ACL/RuleManager.php:112
/home/jld3103/src/github.com/nextcloud/server/apps/groupfolders/lib/ACL/ACLManager.php:227
/home/jld3103/src/github.com/nextcloud/server/apps/groupfolders/lib/Trash/TrashBackend.php:333
/home/jld3103/src/github.com/nextcloud/server/apps/groupfolders/lib/Trash/TrashBackend.php:56
/home/jld3103/src/github.com/nextcloud/server/apps/groupfolders/tests/Trash/TrashBackendTest.php:203

It uses the createUser() method of the UserTrait which wasn't properly initializing the parent by calling the parent constructor.

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)