fix throw RejectedExecutionException

[icodening]

What is the purpose of the change

ref #12632

fix throw RejectedExecutionException when ThreadlessExecutor has been shutdown

Brief changelog

Verifying this change

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
• GitHub Actions works fine on your own branch.
• If this contribution is large, please follow the Software Donation Guide.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

80.0% Coverage
0.0% Duplication

[codecov-commenter]

Codecov Report

Merging #12674 (4163db0) into 3.2 (96fb4e8) will decrease coverage by 0.54%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.2   #12674      +/-   ##
============================================
- Coverage     69.95%   69.42%   -0.54%     
+ Complexity      316        2     -314     
============================================
  Files          1852     1629     -223     
  Lines         97786    67411   -30375     
  Branches      17868     9888    -7980     
============================================
- Hits          68408    46798   -21610     
+ Misses        23893    16096    -7797     
+ Partials       5485     4517     -968     

see 3481 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[EarthChen]

LGTM