Adjusting cluster invoker checks

The current check on cluster invoker is placed in the corresponding implementation class, and almost all implementation classes of AbstractClusterInvoker will call this method.
for example:

However, when the cluster invokers are empty, the subsequent load balancing process is useless.

So I think the check for clustered invokers should be adjusted。

Start by locating AbstractClusterInvoker and the implementation classes that call its cluster-invoker check. Compare the current check with the subsequent load-balancing path shown in the issue. Done means empty cluster invokers are detected before unnecessary load balancing, with the surrounding behavior preserved.

Written by the indexing model from the issue text.