title: Expression Language Symfony Component - cannot validate empty string
body: <p>I tried to play around Expression Language in validation, and I found something strange.</p>
  
  <p>Let's consider Dog model</p>
  
  <pre><code>class Dog
  {
  
      /**
       * @Assert\Expression("this.getName() != 'aa' ", message="Not good!")
       */
      private $name = 'aa';
      ...
  }
  </code></pre>
  
  <p>'aa' != 'aa' => false so validation service reports error - cool,
  but lets try to do the same with </p>
  
  <pre><code>class Dog
  {
  
      /**
       * @Assert\Expression("this.getName() != '' ", message="Not good!")
       */
      private $name = '';
      ...
  }
  </code></pre>
  
  <p>This one does not return any error during validation??? Why? </p>