title: Symfony: Choicetype Field not returning in Request when no option is selected
body: <p>I want to get in the Request the field with an empty value, null or [ ] or whatever.</p>
  <p>The form field looks like this</p>
  <pre class="lang-php prettyprint-override"><code>-&gt;add('preferencia', ChoiceType::class, [
      'choices'  =&gt; [
          'Catamaranes de vela' =&gt; 'Vela',
          'Catamaranes de motor' =&gt; 'Motor',
          'Trimaranes' =&gt; 'Trimaranes',
      ], 
      'expanded' =&gt; true,
      'multiple' =&gt; true,
      'mapped' =&gt; false,
      'required' =&gt; false,
  ])
  </code></pre>
  <p>And I think that is all the necessary code. Just in case, in the controller I get the submitted result with</p>
  <pre class="lang-php prettyprint-override"><code>$result = $request-&gt;request-&gt;all(&quot;curriculo_nautico_form&quot;);
  </code></pre>
  <p>When the form is submitted without any choice selected,  <code>$result</code> is missing that field, I want to still return the field but with an empty array or whatever. Like</p>
  <pre class="lang-php prettyprint-override"><code>$result['preferencia'] = []||null||something
  </code></pre>
  <p>I know I can just check manually if the field was retrieved, but I'm using this type of field several times so I want to automate it.</p>
  <p>Thank you in advance :)</p>