fix: toCommaDelimitedString returns "null,null" when inputs are null

[ankitshokeen]

Issue

#15321

What is the purpose of the change?

This change corrects the behavior of StringUtils.toCommaDelimitedString(String one, String... others) to return null when either argument is null, as described in its Javadoc. Previously, the method would return a string like "null,null", which could lead to misleading outputs and test failures. This update ensures consistency between the method's implementation and its documented contract.

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 68.53933% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.61%. Comparing base (7b34ad9) to head (328d945).
⚠️ Report is 148 commits behind head on 3.3.

⚠️ Current head 328d945 differs from pull request most recent head 0a2d0e3

Please upload reports for the commit 0a2d0e3 to get more accurate results.

Files with missing lines	Patch %	Lines
...stry/client/ServiceDiscoveryRegistryDirectory.java	70.21%	10 Missing and 4 partials ⚠️
...moting/transport/netty4/NettyConnectionClient.java	40.00%	8 Missing and 1 partial ⚠️
.../dubbo/registry/integration/RegistryDirectory.java	88.00%	1 Missing and 2 partials ⚠️
...ava/org/apache/dubbo/common/utils/StringUtils.java	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (7b34ad9) and HEAD (328d945). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (7b34ad9)	HEAD (328d945)
unit-tests	1	0

Additional details and impacted files

@@              Coverage Diff              @@
##                3.3   #15322       +/-   ##
=============================================
- Coverage     60.76%   35.61%   -25.15%     
+ Complexity    10918    10882       -36     
=============================================
  Files          1886     1886               
  Lines         86127    86163       +36     
  Branches      12903    12909        +6     
=============================================
- Hits          52334    30688    -21646     
- Misses        28332    51004    +22672     
+ Partials       5461     4471      -990     

Flag	Coverage Δ
integration-tests	32.97% <67.41%> (-0.23%)	⬇️
samples-tests	29.40% <68.53%> (+0.06%)	⬆️
unit-tests	?

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[ankitshokeen]

@AlbumenJ @songxiaosheng @JunJieLiu51520 please review this PR

[oxsean]

Why are there so many unrelated changes?

[ankitshokeen]

Why are there so many unrelated changes?

Invalid PR, please ignore
working on another.

[ankitshokeen]

Why are there so many unrelated changes?

@oxsean please check #15338