[Dubbo-4218] Fix NPE when the TagRouterRule addresses config is null (#4218)

[web1992]

What is the purpose of the change

Fix NPE when the TagRouterRule addresses config is null #4218

Brief changelog

Fix NPE when the TagRouterRule addresses config is null #4218

Verifying this change

XXXXX

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[slievrly]

why import *?

[web1992]

Suggested change

	import java.util.*;
	import java.util.*;

fix it

[slievrly]

assert tagRouterRule.getAddresses().size==1

[web1992]

I add it

[codecov-io]

Codecov Report

Merging #4236 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #4236      +/-   ##
============================================
- Coverage     63.01%   62.92%   -0.09%     
- Complexity      505      506       +1     
============================================
  Files           769      769              
  Lines         33058    33061       +3     
  Branches       5217     5217              
============================================
- Hits          20830    20804      -26     
- Misses         9823     9853      +30     
+ Partials       2405     2404       -1

Impacted Files	Coverage Δ	Complexity Δ
...bo/rpc/cluster/router/tag/model/TagRouterRule.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	0% <0%> (ø)
.../remoting/transport/netty4/NettyServerHandler.java	61.9% <0%> (-9.53%)	0% <0%> (ø)
.../apache/dubbo/remoting/transport/AbstractPeer.java	60.86% <0%> (-6.53%)	0% <0%> (ø)
...ng/transport/dispatcher/all/AllChannelHandler.java	51.42% <0%> (-5.72%)	0% <0%> (ø)
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0%> (-5.27%)	0% <0%> (ø)
.../org/apache/dubbo/remoting/ExecutionException.java	15.78% <0%> (-5.27%)	0% <0%> (ø)
...exchange/support/header/HeaderExchangeHandler.java	64.75% <0%> (-2.46%)	0% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	67.87% <0%> (-1.81%)	0% <0%> (ø)
...dubbo/metadata/support/AbstractMetadataReport.java	71.09% <0%> (-1.74%)	0% <0%> (ø)
... and 6 more

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[CrazyHZM]

LGTM