fix(VCombobox): v-combobox placeholder is not consistent with v-textbox

[giuliohome]

fixes #20660

Description

See my comment here.

Markup:

<template>
  <v-app>
    <v-container>
      <v-text-field v-model="msg" placeholder="Enter the message" />

      <v-combobox v-model="msg" placeholder="Enter the message" />
    </v-container>
  </v-app>
</template>

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

  const msg = ref('')
</script>

<script>
  export default {
    name: 'Playground',
    setup () {
      return {
        //
      }
    },
  }
</script>