fix(VItemGroup): allow null values

[KaelWD]

Description

fixes #9073
I couldn't find any justification for this in #5009. There is a chance this change could break anything relying on that behaviour but it seems pretty obtuse.

Markup:

<template>
  <v-app>
    <v-container>
      <v-radio-group v-model="selected">
        <v-radio
          label="Yes"
          :value="true"
        ></v-radio>
        <v-radio
          label="No"
          :value="false"
        ></v-radio>
        <v-radio
          label="N/A"
          :value="null"
        ></v-radio>
      </v-radio-group>

      <v-tabs>
        <v-tab>Tab</v-tab>
        <v-tab>Tab</v-tab>
        <v-tab>Tab</v-tab>
      </v-tabs>

      <pre>{{ String(selected) }}</pre>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    data: () => ({
      selected: undefined,
    }),
  }
</script>