fix(runtime-dom): clear prev style with string and object

[FireBushtree]

1. use this code, after click button, the text style will change to 'font-size: 12px'

<script setup>
import { ref } from 'vue'

const style = ref({ color: 'red' })
function handleClick() {
  style.value = { fontSize: '12px' }
}
</script>

<template>
  <h1 :style="style">Hello World</h1>
  <button @click="handleClick">change style</button>
</template>

2. but this code, after click button, the text style will change to 'color: red; font-size: 12px'. the 'color: red' do not be removed

<script setup>
import { ref } from 'vue'

const style = ref('color: red')
function handleClick() {
  style.value = { fontSize: '12px' }
}
</script>

<template>
  <h1 :style="style">Hello World</h1>
  <button @click="handleClick">change style</button>
</template>

[yyx990803]

Thanks for the PR - however I think #10373 is a cleaner fix as it avoids allocating a temporary object (we only need the keys).