title: Symfony Validation Constraint Assert\Date() is accepting empty string
body: <p>In an Entity class I defined a property like</p>
  
  <pre><code>use Symfony\Component\Validator\Constraints as Assert;
  
  class MyEntity
  {
      ...
  
      /**
       * @Assert\Date()
       */
      protected $date_of_birth;
  
      ...
  }
  </code></pre>
  
  <p>In the FormType class I use the property:</p>
  
  <pre><code>$builder
      -&gt;add('date_of_birth',
          DateType::class,
          array(
              'label' =&gt; 'Birthday',
              'required' =&gt; true,
              'translation_domain' =&gt; 'messages',
              'widget' =&gt; 'single_text',
              'html5' =&gt; false,
              'data' =&gt; null,
              'format' =&gt; $this-&gt;container-&gt;getParameter('date.format.icu'),
              'attr' =&gt; array( 'class' =&gt; 'js-bootstrap-datepicker', 'placeholder' =&gt; 'DD.MM.YYYY' ),
          )
      )
  ;
  </code></pre>
  
  <p>In the template I output the form suppressing browser side validation:</p>
  
  <pre><code>...
  {{ form_start(form_a, {'attr': {'novalidate': 'novalidate'}}) }}
  ...
  {{ form_label(form_a.date_of_birth) }}*
  {{ form_errors(form_a.date_of_birth) }}
  {{ form_widget(form_a.date_of_birth) }}
  ...
  </code></pre>
  
  <p>Unfortunately it doesn't really matter what I type into the date field. I can leave it empty - it still is OK for the validator.
  The NotBlank Constraint works though.
  I suppose I am missing something specific in the Date Constraint usage?</p>