Fix NPE in NettyConnectionClient.toString

[AlbumenJ]

What is the purpose of the change

Fix https://github.com/apache/dubbo/actions/runs/8319245558/job/22762468328

Brief changelog

Verifying this change

Checklist

• [x] 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.

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.49%. Comparing base (0553d70) to head (2f568a4). Report is 4 commits behind head on 3.2.

Additional details and impacted files

@@            Coverage Diff             @@
##              3.2   #13934      +/-   ##
==========================================
- Coverage   70.56%   70.49%   -0.08%     
==========================================
  Files        1607     1607              
  Lines       70071    70073       +2     
  Branches    10100    10099       -1     
==========================================
- Hits        49449    49395      -54     
- Misses      16010    16053      +43     
- Partials     4612     4625      +13     

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