fix(VCombobox): hide placeholder when dirty

[johnleider]

Motivation and Context

fixes #16519
fixes #16493

Markup:

<template>
  <v-app>
    <v-main>
      <v-autocomplete
        v-model="item"
        placeholder="autocomplete"
        :persistent-placeholder="placeholder"
        :items="['item1','item2']"
        :multiple="multiple"
        :label="label ? 'Label' : undefined"
      />
      <v-combobox
        v-model="item"
        placeholder="combobox"
        :persistent-placeholder="placeholder"
        :items="['item1','item2']"
        :multiple="multiple"
        :label="label ? 'Label' : undefined"
      />
      <v-select
        v-model="item"
        :persistent-placeholder="placeholder"
        :items="['item1','item2']"
        placeholder="select"
        :multiple="multiple"
        :label="label ? 'Label' : undefined"
      />
      <v-text-field
        v-model="msg"
        :persistent-placeholder="placeholder"
        placeholder="text-field"
        :label="label ? 'Label' : undefined"
      />
      <v-textarea
        v-model="msg"
        :persistent-placeholder="placeholder"
        placeholder="text-area"
        :label="label ? 'Label' : undefined"
      />
      <v-switch v-model="label" label="label" />
      <v-switch v-model="multiple" label="multiple" />
      <v-switch v-model="placeholder" label="persist placeholder" />
      <v-btn @click="toggle">Toggle values</v-btn>
    </v-main>
  </v-app>
</template>

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

  const msg = ref('')
  const item = ref()
  const multiple = ref(true)
  const label = ref(false)
  const list = ref(['A', 'B', 'C'])
  const placeholder = ref(false)

  function toggle () {
    if (item.value) item.value = null
    else {
      item.value = multiple.value
        ? list.value
        : list.value[0]
    }
  }
</script>

[marjev]

I believe my PR (#17115) should be closed in favor of this one.

[6XGate]

Does this fix #17113 ?