fix: set is dirty false if empty string to show the placeholder (#21972)

[jcjp]

Description

• Fixes: #21972
• Check the model value if it's an empty string and treat it as a not dirty value
• In this way we could show the placeholder from props

Markup:

<template>
  <v-app>
    <v-container>
      <v-combobox
        v-model="d"
        placeholder="placeholder"
        persistent-placeholder
        :items="['a','b','c']"
      />
    </v-container>
  </v-app>
</template>

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

  const d = ref('')
</script>

[J-Sek]

It should rather be named isArrayWithEmptyString. Regardless... I don't think a combobox with multiple and model-value="['']" can be considered empty. It behaves differently when selecting more values. Showing placeholder in this case could be misleading.

[jcjp]

Oh okay gotcha, so this should be regarded as intended.