title: Vuetify v-combobox validations not working
body: <p>I have a Vuetify(v. 1.9.0) <code>v-combobox</code> with validation that at least one item should be selected from the menu. For some reason the validation is not triggered and there are no errors in the console.</p>
  
  <pre><code>&lt;v-combobox
    multiple
    item-text="name"
    item-value="id"
    :items="items"
    :rules="[rules.required]"
  &gt;&lt;/v-combobox&gt;
  
  &lt;script&gt;
    export default {
       data: () =&gt; ({
         rules: {
           required: value =&gt;
             !!value || "Required."
         }
       })
    }
  &lt;/script&gt;
  </code></pre>
  
  <p>What am I missing?</p>