fail PekkoDiscoveryNameResolver on invalid addresses

[raboof]

ResolvedTarget has an address: Option[InetAddress]. PekkoDiscoveryNameResolver uses that, even when its .getAddress returns null. It would probably be better to fail early and treat this the same as a target with a host without an address that fails to resolve?

(if we change this we probably need to adapt some tests around balancing to test with 'valid but non-responsive' addresses rather than 'invalid' addresses)