fix(VSelects): recognize empty string as no value

[J-Sek]

Description

Selection controls should correctly reflect empty state. And most of the times empty string is perceived by end users as "no value". There is an edge case if "" is among options' values - it results in selection in VSelection and VAutocomplete, while VCombobox will not recognize it as selection because it does not care about item value (thus has hardcoded [''] and won't consume emptyValues from useItems)

fixes #20660

Markup:

<template>
  <v-app theme="dark">
    <v-container>
      <v-row>
        <v-col>
          <h4>Typical options</h4>
          <v-divider class="my-3" />
          <h5>VTextField</h5>
          <v-text-field v-model="msg1" :items="options1" clearable placeholder="Enter the message" />
          <h5>VSelect</h5>
          <v-select v-model="msg1" :items="options1" clearable placeholder="Select from the options" />
          <h5>VCombobox</h5>
          <v-combobox v-model="msg1" :items="options1" clearable placeholder="Select or type custom value" />
          <h5>VAutocomplete</h5>
          <v-autocomplete v-model="msg1" :items="options1" clearable placeholder="Type to find an item" />

          <pre>{{ JSON.stringify({ msg1 }) }}</pre>
        </v-col>
        <v-col>
          <h4>Options with empty string value</h4>
          <v-divider class="my-3" />
          <h5>VTextField</h5>
          <v-text-field v-model="msg2" :items="options2" clearable placeholder="Enter the message" />
          <h5>VSelect</h5>
          <v-select v-model="msg2" :items="options2" clearable placeholder="Select from the options" />
          <h5>VCombobox</h5>
          <v-combobox v-model="msg2" :items="options2" clearable placeholder="Select or type custom value" />
          <h5>VAutocomplete</h5>
          <v-autocomplete v-model="msg2" :items="options2" clearable placeholder="Type to find an item" />

          <pre>{{ JSON.stringify({ msg2 }) }}</pre>
        </v-col>
      </v-row>
    </v-container>
  </v-app>
</template>

<script setup>
import { ref } from 'vue'
const msg1 = ref('')
const msg2 = ref('')

const options1 = [
  { title: 'Alabama', value: 'AL' },
  { title: 'Alaska', value: 'AK' },
  { title: 'American Samoa', value: 'AS' },
  { title: 'Arizona', value: 'AZ' },
  { title: 'Arkansas', value: 'AR' },
]
const options2 = [
  { title: "(I don't want to disclose)", value: '' },
  { title: 'California', value: 'CA' },
  { title: 'Colorado', value: 'CO' },
  { title: 'Connecticut', value: 'CT' },
  { title: 'Delaware', value: 'DE' },
]
</script>