Remove `return None` for get_physical_link()
Remove `return None` for get_physical_link()

The function get_physical_link() will return None by default.

Change-Id: Ia04c147c02129d860c3302f964b30a8377e64035

stackforge/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard