policy: NULL path isn't really acceptable - use the devname

According to:
	commit b451aa4846c5ccca5447a6b6d45e5623b8c8e961
	Fix handling for "auto" line in mdadm.conf

a NULL path isn't really acceptable and the devname should be used instead.

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>