title: Compile Error: Cannot use isset() on the result of an expression
body: <p>I'm getting this error in a app I am migrating from SF2.0.x to SF2.7:</p>
  
  <pre><code>[1] Symfony\Component\Debug\Exception\FatalErrorException: Compile Error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
      at n/a
          in /var/www/html/reptooln_admin/app/cache/dev/twig/68/7f/63589dd3687cb849dd68e6b6c10aa99eda1d82f95a5f3ac52a864d200499.php line 39
  </code></pre>
  
  <p>I don't know what is failing or how to fix this so I need some advise. This is the line at cache file where the Stacktrace is reported:</p>
  
  <pre><code>    if ((((empty((isset($context["form_action"]) ? $context["form_action"] : $this-&gt;getContext($context, "form_action"))) == true) || (isnull((isset($context["form_action"]) ? $context["form_action"] : $this-&gt;getContext($context, "form_action"))) == true)) || (isset((isset($context["form_action"]) ? $context["form_action"] : $this-&gt;getContext($context, "form_action"))) == false))) {
                  echo " ";
                  $context["form_action"] = "";
                  echo " ";
  </code></pre>
  
  <p>What I have this TwigExtension:</p>
  
  <pre><code>class PDOneTwigExtension extends \Twig_Extension
  {
      public function getFilters()
      {
          return array(
              'var_dump'   =&gt; new \Twig_Filter_Function('var_dump'),
              'empty' =&gt; new \Twig_Filter_Function('empty', array($this, 'is_empty')),
              'isset' =&gt; new \Twig_Filter_Function('isset', array($this, 'is_set')),
              'isnull' =&gt; new \Twig_Filter_Function('isnull', array($this, 'is_null')),
              'ucfirst' =&gt; new \Twig_Filter_Function('ucfirst', array($this, 'uc_first')),
              'ucwords' =&gt; new \Twig_Filter_Function('ucwords', array($this, 'uc_words')),
              'count' =&gt; new \Twig_Filter_Function('count', array($this, 'co_unt')),
              'sizeof' =&gt; new \Twig_Filter_Function('sizeof', array($this, 'size_of')),
              'concat' =&gt; new \Twig_Filter_Function('concat', array($this, 'concat')),
              'in_array' =&gt; new \Twig_Filter_Function('in_array', array($this, 'inarray')),
              'array' =&gt; new \Twig_Filter_Function('array', array($this, 'array_')),
              'add_to_array' =&gt; new \Twig_Filter_Function('add_to_array', array($this, 'add_to_array')),
              'replace' =&gt; new \Twig_Filter_Function('replace', array($this, 'replace')),
              'htmlentitydecode' =&gt; new \Twig_Filter_Function('htmlentitydecode', array($this, 'htmlentitydecode'))
          );
      }
  
      public function is_empty($sentence)
      {
          return empty($sentence) ? true : false;
      }
  
      // rest of methods goes here
  
      public function getName()
      {
          return 'pdone_twig_extension';
      }
  }
  </code></pre>
  
  <p>And I'm using at template as follow:</p>
  
  <pre><code>{% if form_action|empty == true or form_action|isnull == true or form_action|isset == false %} {% set form_action = '' %} {% endif %}
  </code></pre>
  
  <p>Where could be the issue here? Any advice?</p>