title: Twig runs in 'if variable is defined', even when variable is defined
body: <p>I want to include a template, when a variable is set. When the variable isn't set, the template must not be included.</p>
  
  <pre><code>{% if data is defined %}
      {% block content %}
          {% include 'data.html.twig' with  { 'data' : data} %} {# Line 14 #}
      {% endblock %}  
  {% endif %}
  </code></pre>
  
  <p>But this check doesn't work. When <code>data</code> isn't defined an error occures:</p>
  
  <pre><code> Twig_Error_Runtime: Variable "data" does not exist in "text.html.twig" at line 14
  </code></pre>
  
  <p>But Twig has to skip that line when <code>data</code> is defined. Who can explain this behaviour, en more importan: How can I solve this?</p>