Fix StreamObserver object calls onCompleted() without first calling onNext() will fail.

[RainYuY]

What is the purpose of the change?

#15465

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.92%. Comparing base (f7553a0) to head (d4199ae).
⚠️ Report is 131 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
...l/tri/h12/http1/Http1SseServerChannelObserver.java	0.00%	4 Missing ⚠️
...ting/http12/AbstractServerHttpChannelObserver.java	33.33%	1 Missing and 1 partial ⚠️
...l/tri/h12/http2/Http2SseServerChannelObserver.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #15466      +/-   ##
============================================
- Coverage     60.94%   60.92%   -0.02%     
- Complexity    11451    11453       +2     
============================================
  Files          1888     1888              
  Lines         86363    86379      +16     
  Branches      12956    12958       +2     
============================================
- Hits          52633    52627       -6     
- Misses        28298    28309      +11     
- Partials       5432     5443      +11     

Flag	Coverage Δ
integration-tests-java17	32.95% <0.00%> (-0.15%)	⬇️
integration-tests-java8	33.14% <0.00%> (-0.05%)	⬇️
samples-tests-java17	31.43% <0.00%> (-0.03%)	⬇️
samples-tests-java8	29.31% <0.00%> (-0.05%)	⬇️
unit-tests	58.86% <36.36%> (+0.02%)	⬆️

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[RainYuY]

PTAL @oxsean

[oxsean]

Http2SseServerChannelObserver should be update too.

[RainYuY]

Http2SseServerChannelObserver should be update too.

A little diff.PTAL @oxsean