Fix remoteMap may be null to avoid NullPointerException being thrown(#7653)

[skyguard1]

What is the purpose of the change

In this issue(#7653 ) RemoteMap may be null, NullPointerException will be thrown in ClusterUtils, this pr is to fix this issue, to avoid throwing NullPointerException in ClusterUtils, thanks

Brief changelog

Verifying this change

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[codecov-commenter]

Codecov Report

Merging #7654 (cc9ed3a) into master (68062c1) will increase coverage by 0.04%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #7654      +/-   ##
============================================
+ Coverage     59.17%   59.21%   +0.04%     
  Complexity      529      529              
============================================
  Files          1076     1076              
  Lines         43451    43452       +1     
  Branches       6352     6353       +1     
============================================
+ Hits          25711    25731      +20     
+ Misses        14901    14877      -24     
- Partials       2839     2844       +5     

Impacted Files	Coverage Δ	Complexity Δ
...apache/dubbo/rpc/cluster/support/ClusterUtils.java	69.81% <50.00%> (-1.35%)	0.00 <0.00> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	70.17% <0.00%> (-3.51%)	8.00% <0.00%> (-1.00%)
...c/main/java/org/apache/dubbo/rpc/RpcException.java	80.00% <0.00%> (-3.34%)	0.00% <0.00%> (ø%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	61.38% <0.00%> (-2.98%)	0.00% <0.00%> (ø%)
...ava/org/apache/dubbo/config/ServiceConfigBase.java	59.63% <0.00%> (-0.61%)	0.00% <0.00%> (ø%)
...ubbo/registry/client/ServiceDiscoveryRegistry.java	42.26% <0.00%> (-0.60%)	0.00% <0.00%> (ø%)
...ng/transport/dispatcher/all/AllChannelHandler.java	89.65% <0.00%> (ø)	0.00% <0.00%> (ø%)
...apache/dubbo/common/extension/ExtensionLoader.java	78.39% <0.00%> (+0.22%)	0.00% <0.00%> (ø%)
...he/dubbo/registry/multicast/MulticastRegistry.java	69.30% <0.00%> (+0.93%)	0.00% <0.00%> (ø%)
...pache/dubbo/registry/support/AbstractRegistry.java	80.74% <0.00%> (+1.11%)	0.00% <0.00%> (ø%)
... and 3 more

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