ImageService v2: Fix paging when next URL is blank (#254)

diff --git a/openstack/imageservice/v2/images/results.go b/openstack/imageservice/v2/images/results.go
index ae674de..09996f4 100644
--- a/openstack/imageservice/v2/images/results.go
+++ b/openstack/imageservice/v2/images/results.go
@@ -156,6 +156,11 @@
 	if err != nil {
 		return "", err
 	}
+
+	if s.Next == "" {
+		return "", nil
+	}
+
 	return nextPageURL(r.URL.String(), s.Next), nil
 }