fix(VCombobox): don't reset value to null if search is empty

[KaelWD]

Description

fixes #12322

I can't think of a good reason to count '' as clean, search is reset to null on blur anyway so it can only mean:

• value is '' and nothing was changed, or
• value is set and search was deleted by the user

Only checked with combobox, probably doesn't break autocomplete though.

Markup:

<template>
  <v-container>
    <pre>{{ JSON.stringify(String(select)) }}</pre>
    <pre>{{ JSON.stringify(String(search)) }}</pre>
    <v-combobox v-model="select" :search-input.sync="search"></v-combobox>
    <v-btn @click="select = ''">reset</v-btn>
    <v-btn @click="search = ''">reset search</v-btn>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      select: '',
      search: '',
    }),
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

[MajesticPotatoe]

LGTM