Fix exception on external storages public links when encryption enabled

[pako81]

Description

Having masterkey encryption enabled, when trying to upload into a public link created on an external storage (i.e. of type "Local" or "WND") being assigned to at least one user/group, an exception Attempted to initialize mount points for null user and no user in session is being thrown and no upload is possible.

Related Issue

• Fixes https://github.com/owncloud/enterprise/issues/6626

Motivation and Context

There was a problem around https://github.com/owncloud/core/blob/master/lib/private/Encryption/Util.php#L303 as the uid provided to the method is false in this case. This happens because the external storage doesn't really have an owner and there is no user in the session. Therefore, when checking for isMountPointApplicableToUser, the provided user is false which will never find a match, so the mount point isn't considered system wide.

How Has This Been Tested?

1. enable masterkey encryption
2. create an external storage through the "local" option (also reproducible with WND) and mount it for user admin
3. as user admin creates a public link on the external storage root
4. try to upload into the created public link: an exception Attempted to initialize mount points for null user and no user in session is shown and file cannot be uploaded
5. try to download an existing file works. Try to download any other file added by user admin after the storage has been mounted into oC fails with File cannot be downloaded and same exception in logs
6. With this change in place both exceptions at 4. and 5. are solved.

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Database schema changes (next release will require increase of minor version instead of patch)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Technical debt
• [ ] Tests only (no source changes)

Checklist:

• [X] Code changes
• [ ] Unit tests added
• [ ] Acceptance tests added
• [ ] Documentation ticket raised
• [x] Changelog item