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.

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