Use empty string as default value for Security Group description

* Return SG description field to empty string if none value.

Closes-Bug: #1766508
(cherry picked from commit e5e4322)

Conflicts:
	src/config/vnc_openstack/vnc_openstack/neutron_plugin_db.py

Change-Id: Ic40c614b3bafbd93e50569d4c31fcfb584954e7a

src/config/vnc_openstack/vnc_openstack/neutron_plugin_db.py

@@ -1092,7 +1092,10 @@ def _security_group_vnc_to_neutron(self, sg_obj, memo_req=None):
         else:
             sg_q_dict['name'] = sg_obj.display_name
 
+        # If description is not specified by the user, assign empty string
         sg_q_dict['description'] = sg_obj.get_id_perms().get_description()
+        if not sg_q_dict['description']:
+            sg_q_dict['description'] = ""
 
         # get security group rules
         sg_q_dict['security_group_rules'] = []