fix(components): [select] fix placeholder display

[sleepyShen1989]

closed #11049

动态修改placeholder时，判断modelValue是否有值，如果有值则清空placeholder

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[github-actions]

Hello @sleepyShen1989, 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=11051
Please comment the example via this playground if needed.

[Lionad-Morotar]

@btea Hello, would this pr being merged in the next release?

[btea]

@Lionad-Morotar Also needs to be reviewed by other team members. Please be patient.