Fix neutron port-create --no-security-groups

Previously, this would pass in {'security_groups': null} to neutron-server
which uses is_attr() in _ensure_default_security_group_on_port() which returns
False as is_attr() now checks for None and not just ATTR_NOT_SPECIFIED.

Change-Id: I033ce62dbb91597541a8c7f773b9819d69761860
Closes-bug: 1257473