fix(components): [select] fix option-group get wrong visible

[init-qy]

closed #9711

it can fix this problem, with nextTick I can get current children visible
but I don't know what was modified in vue3.2.38 cause this problem

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[xiaoxian521]

Please add the demo before and after the repair so that we can better review

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=9726
Please comment the example via this playground if needed.

[init-qy]

playground

[init-qy]

nextTick looks like break a test case， so I add it, now it works

[holazz]

Maybe use post flush watcher instead of nextTick?

[init-qy]

Maybe use post flush watcher instead of nextTick?

ok, it aslo works, I first learned about watcher Callback Flush Timing
later I will change it