Dubbogo-cli-v2 show zk with no dubbo service, cause panic.

[LaurenceLiZhixin]

problem 1:

 dubbogo-cli-v2 show --r zookeeper --h 127.0.0.1:2181
2022/03/16 16:27:53 Connected to 127.0.0.1:2181
2022/03/16 16:27:53 Authenticated: id=72060066796601349, timeout=15000
2022/03/16 16:27:53 Re-submitting `0` credentials after reconnect
panic: Error while invoking zk.Children(path:/dubbo), the reason maybe is: : zk: node does not exist

problem2:

Dubbogo-cli-v2 show interface twice?

dubbogo-cli-v2 show --r zookeeper --h 127.0.0.1:2181
2022/03/16 16:29:35 Connected to 127.0.0.1:2181
2022/03/16 16:29:35 Authenticated: id=72060066796601357, timeout=15000
2022/03/16 16:29:35 Re-submitting `0` credentials after reconnect
interface: grpc.reflection.v1alpha.ServerReflection
methods: [ServerReflectionInfo]
interface: org.apache.dubbo.demo.Greeter
methods: [SayHello,SayHelloStream]
interface: grpc.reflection.v1alpha.ServerReflection
methods: [ServerReflectionInfo]
interface: org.apache.dubbo.demo.Greeter
methods: [SayHello,SayHelloStream]

[LaurenceLiZhixin]

@jasondeng1997