[SCB-2343]fix highway empty string serialization problem

[liubao68]

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[codecov-commenter]

Codecov Report

Merging #2597 (761695f) into master (ad0dc90) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #2597      +/-   ##
============================================
+ Coverage     77.69%   77.72%   +0.02%     
- Complexity     1428     1429       +1     
============================================
  Files          1598     1598              
  Lines         42626    42652      +26     
  Branches       3591     3591              
============================================
+ Hits          33119    33150      +31     
+ Misses         8006     8000       -6     
- Partials       1501     1502       +1     

Impacted Files	Coverage Δ
...vicecomb/demo/pojo/client/CodeFirstPojoClient.java	98.31% <100.00%> (+0.22%)	⬆️
...pache/servicecomb/demo/pojo/client/PojoClient.java	82.60% <100.00%> (ø)
...ringmvc/client/CodeFirstRestTemplateSpringmvc.java	96.02% <100.00%> (+0.16%)	⬆️
.../src/main/java/io/protostuff/ProtobufOutputEx.java	79.94% <100.00%> (-0.06%)	⬇️
.../servicecomb/registry/discovery/DiscoveryTree.java	96.49% <0.00%> (-3.51%)	⬇️
...ecomb/provider/pojo/PojoConsumerMetaRefresher.java	75.60% <0.00%> (-2.44%)	⬇️
...egistry/client/http/ServiceRegistryClientImpl.java	71.26% <0.00%> (+1.14%)	⬆️
...mb/serviceregistry/client/http/RestClientUtil.java	80.55% <0.00%> (+1.85%)	⬆️

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