NS scenarios without VNFs compatibility

Signed-off-by: gcalvino <guillermo.calvinosanchez@altran.com>
diff --git a/osm_nbi/instance_topics.py b/osm_nbi/instance_topics.py
index 29a391c..5d9705b 100644
--- a/osm_nbi/instance_topics.py
+++ b/osm_nbi/instance_topics.py
@@ -132,7 +132,7 @@
 
             # Create VNFR
             needed_vnfds = {}
-            for member_vnf in nsd["constituent-vnfd"]:
+            for member_vnf in nsd.get("constituent-vnfd", ()):
                 vnfd_id = member_vnf["vnfd-id-ref"]
                 step = "getting vnfd id='{}' constituent-vnfd='{}' from database".format(
                     member_vnf["vnfd-id-ref"], member_vnf["member-vnf-index"])