it should not be tried to create Skeleton for guest users

[davitol]

Install guests app and create a new guest
With any local user, share a folder with a guest user

Steps to reproduce

1. Install guests app and create a new guestCr
2. Create a guest user sharing a folder using an email mail@mymail.com

Expected behavior

It should not be tried to create skeleton files for guest users.

Actual behavior

A log is spotted in owncloud.log

{"reqId":"7KOeew8GHqOhrG3UPttZ","level":2,"time":"2019-11-27T11:00:06+00:00","remoteAddr":"172.25.0.1","user":"dtoledo@solidgeargroup.com","app":"no app in context","method":"POST","url":"\/login","message":"Skeleton not created due to missing write permission"}

Tested with oC 10.3.2 RC1 and Guests 0.8.2. It is NOT A REGRESSION.