fix(VCombobox): don't reset search when cleared

[ElijahKotyluk]

Description

fixes #14507

Motivation and Context

How Has This Been Tested?

visually | unit

Markup:

// Paste your FULL Playground.vue here
<template>
  <v-app>
    input1 @input events={{input1}}
    <v-combobox :items="items" @input="input1++"/>

    input2 @input events={{input2}}
    <v-combobox :items="items" multiple @input="input2++"/>
  </v-app>
</template>

<script>
  export default {
    data: () => ({
      input1: 0,
      input2: 0,
      items: [ "item1", "item2", "item3", "item4"]
    }),
  }
</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)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)