[TwigBridge] fixed the check for the collection prototype in the form theme

src/Symfony/Bridge/Twig/Resources/views/Form/div_layout.html.twig

@@ -263,7 +263,7 @@
 {% block collection_widget %}
 {% spaceless %}
     {{ block('form_widget') }}
-    {% if prototype %}
+    {% if prototype is defined %}
     <script type="text/html" id="{{ id }}_prototype">{{ form_row(prototype) }}</script>
     {% endif %}
 {% endspaceless %}