title: Avoid Symfony forcing form fields display
body: <p>I have a form type, setting 10 fields.
  In my twig template, I display 8 fields everytime, and 2 fields inside a condition :</p>
  
  <pre><code>{% if computed is not empty %}
      &lt;div class="container-filters"&gt;
          &lt;div class="form-group"&gt;
               {{ form_row(form.filterStart) }}
          &lt;/div&gt;
          &lt;div class="form-group"&gt;
                {{ form_row(form.filterEnd) }}
          &lt;/div&gt;
          &lt;input type="submit" name="submit" class="btn btn-evo" value="OK" /&gt;
      &lt;/div&gt;
  {% endif %}
  </code></pre>
  
  <p>But no matter if my condition is true or false, Symfony will display those 2 fields. If my condition is true, they will be displayed because of my {{ form_row() }} statement, which is normal behavior. If my condition is false, they will be displayed anyway, because Symfony 2 force form fields to be displayed.</p>
  
  <p>How can I prevent this weird behavior ?</p>