MB-8337: handle 1.8.1 nodes in server details

We don't have storage conf for 1.8.x nodes on UI which was causing
exception on UI. New behavior is to just show empty disk path instead
of showing nothing at all.

Change-Id: I8c76dd985ea55fdf84435ed2f2097f362dc9f6e6
Reviewed-on: http://review.couchbase.org/26572
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>