prop 312: Don't publish IPv6 address if ORPort is IPv4Only
Now that #40025 (closed) is merged, I believe there is an issue for which we assume we'll find an ORPort if we have found an IPv6 address.
In router_build_fresh_unsigned_routerinfo(), we should check at the returned value of router_get_advertised_or_port_by_af() and not publish a port value of 0. It is possible for instance if the ORPort is IPv4Only.