Fix Empty return type in triple generator

[gaoxiaolei-s59]

What is the purpose of the change

Fix the Dubbo3 Triple generator output for unary methods whose response type is google.protobuf.Empty. The generated synchronous Java method now uses void, while descriptor, parser, async, and StreamObserver paths keep using com.google.protobuf.Empty.

Fixes #15629

Brief changelog

• Map google.protobuf.Empty to com.google.protobuf.Empty in generator type context.
• Split unary synchronous return type from protobuf wire output type.
• Add Triple generator coverage for Empty responses.

Verifying this change

• ./mvnw -pl dubbo-plugin/dubbo-compiler test

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.97%. Comparing base (c3ceb27) to head (f5889a1).

Files with missing lines	Patch %	Lines
...n/java/org/apache/dubbo/gen/AbstractGenerator.java	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16324      +/-   ##
============================================
+ Coverage     60.79%   60.97%   +0.18%     
+ Complexity    11777    11773       -4     
============================================
  Files          1953     1953              
  Lines         89208    89217       +9     
  Branches      13458    13460       +2     
============================================
+ Hits          54234    54401     +167     
+ Misses        29403    29218     -185     
- Partials       5571     5598      +27     

Flag	Coverage Δ
integration-tests-java21	32.16% <0.00%> (+0.03%)	⬆️
integration-tests-java8	32.25% <0.00%> (-0.08%)	⬇️
samples-tests-java21	32.16% <0.00%> (+0.04%)	⬆️
samples-tests-java8	29.85% <0.00%> (-0.02%)	⬇️
unit-tests-java11	59.21% <90.90%> (+0.18%)	⬆️
unit-tests-java17	58.69% <90.90%> (+0.17%)	⬆️
unit-tests-java21	58.73% <90.90%> (+0.22%)	⬆️
unit-tests-java25	58.67% <90.90%> (+0.19%)	⬆️
unit-tests-java8	59.24% <87.50%> (+0.16%)	⬆️

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[zrlw]

@heliang666s PTAL