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

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 35.65%. Comparing base (c393896) to head (df5f6d4).
Report is 17 commits behind head on 3.3.

❗ Current head df5f6d4 differs from pull request most recent head 4dc4e28

Please upload reports for the commit 4dc4e28 to get more accurate results.

Files with missing lines	Patch %	Lines
...ava/org/apache/dubbo/common/utils/StringUtils.java	0.00%	4 Missing ⚠️

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HEAD has 1 upload less than BASE

Flag	BASE (c393896)	HEAD (df5f6d4)
unit-tests	1	0

Additional details and impacted files

@@              Coverage Diff              @@
##                3.3   #15338       +/-   ##
=============================================
- Coverage     60.76%   35.65%   -25.12%     
+ Complexity    10916    10901       -15     
=============================================
  Files          1886     1886               
  Lines         86127    86130        +3     
  Branches      12903    12904        +1     
=============================================
- Hits          52335    30707    -21628     
- Misses        28335    50972    +22637     
+ Partials       5457     4451     -1006     

Flag	Coverage Δ
integration-tests	33.11% <0.00%> (-0.01%)	⬇️
samples-tests	29.23% <0.00%> (-0.13%)	⬇️
unit-tests	?

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[AlbumenJ]

if one != null && others == null

should return one?

[ankitshokeen]

yes, it should return one