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 or public key user can cause issues.
Merge pull request #12131 from owncloud/stable7-enc-recoverykeywithextstoragefix

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