Make security group description optional in the client.sls

Related prod: PROD-11917

Change-Id: Iebc4f7d095cee2238bc181bf94bb954737a760cc
diff --git a/_states/neutronng.py b/_states/neutronng.py
index 89e0011..afca39b 100644
--- a/_states/neutronng.py
+++ b/_states/neutronng.py
@@ -409,7 +409,7 @@
 
 def security_group_present(name=None,
                            tenant=None,
-                           description=None,
+                           description='',
                            rules=[],
                            profile=None,
                            endpoint_type=None):