Channelz requests for subchannel information should be able to handle `null` Socket address values

The `Socket` field `remote` [is explicitly allowed to be absent](https://github.com/grpc/grpc/blob/master/src/proto/grpc/channelz/channelz.proto#L236), but the code to handle it [unconditionally passes it to `prettyAddress`](https://github.com/grpc-ecosystem/grpcdebug/blob/main/cmd/channelz.go#L74), which [panics if the value is `nil`](https://github.com/grpc-ecosystem/grpcdebug/blob/main/cmd/channelz.go#L40). It would also be good to handle the field `local` similarly, because output that shows that it is `nil` (incorrectly) would be more useful than a panic.