Fix sharing of ext storage mount points

When sharing an ext storage mount point, it will now use the name of the
mount point instead of an empty string for the target path.
Added unit test for sharing ext storage mount points
Fix getPathById for Oracle

Added extra code to handle the case of Oracle which saves empty strings
as null values.
Merge pull request #8499 from owncloud/shareextstoragemountpointfix

Fix sharing of ext storage mount points