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.
Show warning when invalid user was passed

Sometimes there are bugs that cause setupFS() to be called for
non-existing users. Instead of failing hard and breaking the instance,
this fix simply logs a warning.

Backport c941c3fa5139ba4f122d4f40d9c9db5e50f8bcb7 from master
Merge pull request #12131 from owncloud/stable7-enc-recoverykeywithextstoragefix

[stable7] Fix file upload to ext storage when recovery key is enabled
Merge branch 'stable7' of https://github.com/owncloud/core into stable7
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