Issue Description

Type: bug report

Describe what happened (or what feature you want)

Dns-f will return no host found when all instances selected from Selector are disabled。

Describe what you expected to happen

If all ip selected from Selector are disabled， the selector should return all instance

How to reproduce it (as minimally and precisely as possible)

Tell us your environment

Nacos 0.8

Anything else we need to know?

I think it can be fixed by this way