optimize generateMethodContent to avoid unnecessary StringBuilder cre…

[aruato]

…ation

What is the purpose of the change?

avoid unnecessary StringBuilder creation

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?

[wcy666103]

ci fixed in apache/dubbo-samples#1195