fix(VAutocomplete, VCombobox): skip subheader for auto-select-first

[J-Sek]

fixes #22398

Markup:

<template>
  <v-app theme="dark">
    <v-container max-width="450">
      <v-autocomplete :items="items" label="v-autocomplete" auto-select-first />
      <v-combobox :items="items" label="v-combobox" auto-select-first />
    </v-container>
  </v-app>
</template>

<script>
  export default {
    data: () => ({
      items: [
        { type: 'subheader', title: 'Group 1' },
        { title: 'Item 1.1', value: 11 },
        { title: 'Item 1.2', value: 12 },
        { title: 'Item 1.3', value: 13 },
        { title: 'Item 1.4', value: 14 },
        { type: 'divider', text: 'or' },
        { type: 'subheader', title: 'Group 2' },
        { title: 'Item 2.1', value: 21 },
        { title: 'Item 2.2', value: 22 },
        { title: 'Item 2.3', value: 23 },
      ],
    }),
  }
</script>