fix(VDatePicker): resolve firstDayOfWeek from locale

[J-Sek]

Description

Follow-up to recently merged #21199 that introduced regression - locale changes should affect firstDayOfWeek, but since 0 is now recognized as valid prop value, all locales are locked at Sunday (possible to reproduce on "nightly" v3.8.1-master.2025-04-14)

Markup:

<template>
  <v-app style="background-color: rgba(var(--v-theme-on-surface), 0.2)">
    <v-container max-width="650">
      <div class="d-flex justify-center align-center ga-3">
        <span class="opacity-70">locale:</span>
        <v-chip-group v-model="currentLocale">
          <v-chip filter label text="en" value="en"></v-chip>
          <v-chip filter label text="pl" value="pl"></v-chip>
          <v-chip filter label text="fr" value="fr"></v-chip>
          <v-chip filter label text="pt" value="pt"></v-chip>
        </v-chip-group>
      </div>
      <div class="d-flex justify-center align-center ga-3">
        <span class="opacity-70">startOfWeek:</span>
        <v-chip-group v-model="startOfWeek">
          <v-chip
            v-for="o in startOfWeekOptions"
            v-bind="o"
            :key="o.value"
            filter
            label
          ></v-chip>
        </v-chip-group>
      </div>
      <v-alert v-if="startOfWeek === undefined" class="my-3" type="warning" text="First day of week is expected to respond to locale changes" />
      <v-alert v-else class="my-3" type="info" text="First day of week is expected to be locked with selection above" />
      <div class="d-flex justify-center">
        <v-locale-provider :locale="currentLocale">
          <v-date-picker :first-day-of-week="startOfWeek" hide-header />
        </v-locale-provider>
      </div>
    </v-container>
  </v-app>
</template>

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

  const currentLocale = ref('en')
  const startOfWeek = ref(undefined)
  const startOfWeekOptions = 'Sun|Mon|Tue|Wed|Thu|Fri|Sat'
    .split('|')
    .map((x, i) => ({ text: x, value: i }))
</script>