[stable7] Fix file upload to ext storage when recovery key is enabled

Fixes an issue when uploading files to external storage when recovery
keys are enabled

The Util class only works with real users, so instantiating it with the
virtual recovery key user can cause issues.

Fixes https://github.com/owncloud/core/issues/12122

stable7 fix/backport, please confirm @karlitschek

Please review @schiesbn @icewind1991
The user can also be the "public link share user", probably you want to skip this user as well?