optimize MockDirInvocation setObjectAttachmentIfAbsent method code（#11526）

[hardyfish]

What is the purpose of the change

#11526 11526

Brief changelog

optimize MockDirInvocation setObjectAttachmentIfAbsent method code

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.
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• 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.
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[hardyfish]

please check it.

[AlbumenJ]

LGTM