fix(components): [tree-v2] add missing icon

[init-qy]

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close #10339

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[pull-request-triage]

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[github-actions]

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Hello @init-qy, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=10340
Please comment the example via this playground if needed.

[init-qy]

I added some code to test the render of the default icon. Do I need to create a new pr? @holazz

[holazz]

I added some code to test the render of the default icon. Do I need to create a new pr? @holazz

Feel free to create new PRs 😊