fix(group): use index as value if not provided

[johnleider]

Motivation and Context

closes #19107

Markup:

<template>
  <v-app>
    <v-container>
      <pre>{{ JSON.stringify(step, null, 2) }}</pre>
      <v-stepper v-model="step">
        <v-stepper-header>
          <v-stepper-item title="step 1" />
          <v-stepper-item title="step 2" />
        </v-stepper-header>

        <v-stepper-window>
          <v-stepper-window-item> step 1 </v-stepper-window-item>
          <v-stepper-window-item> step 2 </v-stepper-window-item>
        </v-stepper-window>
      </v-stepper>
    </v-container>
  </v-app>
</template>
<script setup>
  import { ref } from 'vue'
  const step = ref(1)
</script>