fixed null value validation for snapshot id
fixed null value validation for snapshot id

jcsp/manila,openstack/manila,redhat-openstack/manila,weiting-chen/manila,scality/manila,jcsp/manila,aostapenko/manila,bswartz/manila,scality/manila,NetApp/manila,aostapenko/manila,sajuptpm/manila,aostapenko/manila,NetApp/manila,vponomaryov/manila,redhat-openstack/manila,vponomaryov/manila,openstack/manila,sajuptpm/manila,bswartz/manila,weiting-chen/manila