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.

Backport of 28f0e63 from master
Added unit test for sharing ext storage mount points

Backport of f73a168 from master
Fix getPathById for Oracle

Added extra code to handle the case of Oracle which saves empty strings
as null values.

Backport of 05dc694 from master
Merge pull request #8521 from owncloud/stable6-shareextstoragemountpointfix

[stable6] Fix sharing of ext storage mount points