Fix DefaultSerializeClassChecker verification, when the mode is WARN

[wuwen5]

What is the purpose of the change?

Fix #15179

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

All modified and coverable lines are covered by tests ✅

Project coverage is 36.42%. Comparing base (29472c0) to head (599cb9f).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.2   #15181      +/-   ##
============================================
- Coverage     41.39%   36.42%   -4.98%     
+ Complexity    10511     9215    -1296     
============================================
  Files          1652     1652              
  Lines         71435    71433       -2     
  Branches      10158    10158              
============================================
- Hits          29568    26016    -3552     
- Misses        37734    41479    +3745     
+ Partials       4133     3938     -195     

Flag	Coverage Δ
integration-tests	?
samples-tests	36.42% <ø> (-0.02%)	⬇️

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[wuwen5]

Perhaps it can return null or DenyClass.class. (wuwen5@d7d3cf6)

If there is a better solution, please let me know.