fix(VCombobox): don't add empty values

[vibhash-singh]

Description

Don't add empty values in Combobox.

Motivation and Context

Fixes #13274

How Has This Been Tested?

Unit testing and Visually

Markup:

<template>
  <div id="app">
    <v-app id="inspire">
      <v-container fluid>
        <v-combobox
          v-model="model"
          :items="items"
          :search-input.sync="search"
          hide-selected
          hint="Maximum of 5 tags"
          label="Add some tags"
          multiple
          persistent-hint
          small-chips
        >
        </v-combobox>
      </v-container>
    </v-app>
  </div>
</template>

<script>
export default {
   data: () => ({
    items: ['Gaming', 'Programming', 'Vue', 'Vuetify'],
    model: ['Vuetify'],
    search: null,
  })
}
</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)