fail PekkoDiscoveryNameResolver on invalid addresses

`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)

Search for PekkoDiscoveryNameResolver and ResolvedTarget, then inspect the resolver path that handles a null getAddress result and the balancing tests mentioned in the issue. Confirm the desired behavior for invalid addresses and adapt coverage to use valid but non-responsive addresses where appropriate.

Written by the indexing model from the issue text.