Only use realpath for real directories (#26058)

In some cross-local-storage use cases, the Local storage is
instantiated with "/" as data directory. In such cases, calling
realpath() would cause PHP warnings when open_basedir is set.

This fix bypasses the realpath() call when dealing with a root storage.

Downstreaming of https://github.com/owncloud/core/pull/26058



Signed-off-by: Lukas Reschke <lukas@statuscode.ch>