title: Vue: v-model undefined or default/fallback
body: <p>Is there anyway to set a default value to an input to the value of another input field in case it has been left empty.
  Looks like I can't specify this in the data nor in the v-model attribute:</p>
  
  <pre><code>&lt;template&gt;
      &lt;div&gt;
          &lt;input type="number" v-model="font1 || 14"&gt; &lt;!-- gives error --&gt;
          &lt;input type="number" v-model="font2 || font1"&gt; &lt;!-- gives error --&gt;
          &lt;input type="number" v-model="font3 || font1"&gt; &lt;!-- gives error --&gt;
      &lt;/div&gt;
  &lt;/template&gt;
  
  &lt;script&gt;
  export default {
      data() {
          return {
              font1: 14,
              font2: this.font2 || this.font1, // this approach also gives error
              font3: this.font3 || this.font1 // this approach also gives error
          }
      }
  }
  &lt;/script&gt;
  </code></pre>
  
  <p>Any help is welcome.
  Thanks.</p>