title: Laravel Nova ignore a field if empty using ->fillUsing()
body: <p>I have 3 fields "name", "email" and "url". These 3 fields are casted to a json in 1 column in my database.</p>
  
  <p>Now if you fill in just the url I want to save only <code>{url: "value"}</code> in the database. If you fill in email and name I want to save only <code>{name: "john", email: "john@gmail.com"}</code> in the database.</p>
  
  <p>This is how I try do to it:</p>
  
  <pre><code>Text::make('To Name', 'toName')
              -&gt;sortable()
              -&gt;fillUsing(
                  function ($request, $model) {
                      return $request-&gt;toName;
                  }
              ),
  
  Text::make('To Email', 'toEmail')
              -&gt;sortable()
              -&gt;fillUsing(
                  function ($request, $model) {
                      return $request-&gt;toEmail;
                  }
              ),
  
  Text::make('To Url', 'toUrl')
              -&gt;sortable()
              -&gt;fillUsing(
                  function ($request, $model) {
                      return $request-&gt;toUrl;
                  }
              ),
  </code></pre>
  
  <p>But I keep getting this error:</p>
  
  <blockquote>
    <p>General error: 1364 Field 'to' doesn't have a default value</p>
  </blockquote>
  
  <p>Am I returning something wrong?</p>