skip network if its id is null (CORD-759)

Change-Id: I65034c5eb7c88f40437c115ca39f8868c61ee69d
diff --git a/xos/vtnnetport.py b/xos/vtnnetport.py
index edb9d21..3f8fac0 100644
--- a/xos/vtnnetport.py
+++ b/xos/vtnnetport.py
@@ -59,6 +59,10 @@
                         continue
 
                     cn = net.controllernetworks.all()[0]
+
+                    if not cn.net_id:
+                        continue
+
                     nets.append({"id": cn.net_id,
                                  "name": net.name,
                                  "bidirectional": bidirectional})
@@ -80,6 +84,10 @@
                         continue
 
                     cn = net.controllernetworks.all()[0]
+
+                    if not cn.net_id:
+                        continue
+
                     nets.append({"id": cn.net_id,
                                  "name": net.name,
                                  "bidirectional": bidirectional})