Fix port command for SDK >0.9.10

The port functional test can not be passed in my local environment.
When 'dns_assignment' is None, the port create, show command will
fail because parameter for 'utils.format_list_of_dicts' can not be
None.

Change-Id: Iebf16fb7ca681660c2b9ee7839a0629f38c6a38a

Author: jiahui.qiang

openstackclient/network/v2/port.py

@@ -35,6 +35,10 @@ def _format_admin_state(state):
     return 'UP' if state else 'DOWN'
 
 
+def _format_dns_assignment(dns_assignment):
+    return utils.format_list_of_dicts(dns_assignment) \
+        if dns_assignment else None
+
 _formatters = {
     'admin_state_up': _format_admin_state,
     'is_admin_state_up': _format_admin_state,
@@ -43,7 +47,7 @@ def _format_admin_state(state):
     'binding_vif_details': utils.format_dict,
     'binding:profile': utils.format_dict,
     'binding:vif_details': utils.format_dict,
-    'dns_assignment': utils.format_list_of_dicts,
+    'dns_assignment': _format_dns_assignment,
     'extra_dhcp_opts': utils.format_list_of_dicts,
     'fixed_ips': utils.format_list_of_dicts,
     'security_group_ids': utils.format_list,