For compatible nacos server lower version, we should check the response from nacos server is null.

[horizonzy]

What is the purpose of the change

Fix #8181

At provider, it won't register service name mapping info.
At consumer, we will use application_first, it will use service discovery logic to find service name mapping info,
in NacosDynamicConfiguration, use httpAgent to get service name mapping info, but at nacos server lower version, if the config is not found, will return null response.

So we should do compatible work for lower nacos server version.

[codecov-commenter]

Codecov Report

Merging #8229 (20b7edb) into master (149d1f6) will decrease coverage by 0.19%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #8229      +/-   ##
============================================
- Coverage     60.99%   60.80%   -0.20%     
+ Complexity      490      456      -34     
============================================
  Files          1090     1090              
  Lines         43951    43955       +4     
  Branches       6423     6421       -2     
============================================
- Hits          26810    26726      -84     
- Misses        14156    14250      +94     
+ Partials       2985     2979       -6     

Impacted Files	Coverage Δ
...enter/support/nacos/NacosDynamicConfiguration.java	0.00% <0.00%> (ø)
...e/dubbo/registry/consul/ConsulRegistryFactory.java	0.00% <0.00%> (-100.00%)	⬇️
...g/apache/dubbo/registry/consul/ConsulRegistry.java	0.00% <0.00%> (-60.00%)	⬇️
...g/apache/dubbo/remoting/p2p/support/FileGroup.java	41.50% <0.00%> (-22.65%)	⬇️
...nfig/configcenter/nop/NopDynamicConfiguration.java	62.50% <0.00%> (-12.50%)	⬇️
...che/dubbo/remoting/transport/mina/MinaChannel.java	35.52% <0.00%> (-10.53%)	⬇️
...ng/transport/dispatcher/all/AllChannelHandler.java	82.75% <0.00%> (-6.90%)	⬇️
.../threadpool/manager/DefaultExecutorRepository.java	80.95% <0.00%> (-4.77%)	⬇️
...ng/transport/dispatcher/WrappedChannelHandler.java	58.69% <0.00%> (-4.35%)	⬇️
...o/rpc/cluster/support/migration/MigrationRule.java	50.00% <0.00%> (-3.58%)	⬇️
... and 44 more

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[xiaoheng1]

LGTM