title: Symfony2 - NotBlank constraint not working on EntityType
body: <p>I have two <code>EntityType</code> fields in my form and both has <code>NotBlank</code> constraint assigned to them. </p>
  
  <p>Now, I have this issue that <code>NotBlank</code> constraint is not working only on one field with <code>multiple =&gt; true</code> set on.</p>
  
  <pre><code>$builder
      -&gt;add('preferredCountries', EntityType::class, array(
          'required' =&gt; false,
          'class' =&gt; 'IndexBundle:Country',
          'property' =&gt; 'name',
          'empty_value' =&gt; 'Choose',
          'multiple' =&gt; true,
          'query_builder' =&gt; function (EntityRepository $er) {
              return $er-&gt;createQueryBuilder('c')
                  -&gt;where('c.name != :name')
                  -&gt;orderBy('c.name', 'ASC')
                  -&gt;setParameter('name', 'Other');
          },
          'constraints' =&gt; array(
              new NotBlank(array(
                  'message' =&gt; 'blank!!!',
              )),
          )
      ))
      -&gt;add('internshipProgram', EntityType::class, array(
          'required' =&gt; false,
          'class' =&gt; 'IndexBundle:InternshipProgram',
          'property' =&gt; 'name',
          'empty_value' =&gt; 'Choose',
          'constraints' =&gt; array(
              new NotBlank(array(
                  'message' =&gt; 'blank!!!',
              )),
          )
      ))
  </code></pre>
  
  <p>In this case when I submit empty values, field <code>internshipProgram</code> get an error, and <code>prefferedCountries</code> not.</p>
  
  <p>Form display:</p>
  
  <pre><code>&lt;div class="form-group col-xs-12"&gt;
      {{ form_label(current_internship_form.preferredCountries, 'Preferred countries', { 'label_attr': {'class': 'label-text'} }) }}
      {{ form_widget(current_internship_form.preferredCountries) }}
      &lt;span class="error text-danger small"&gt;{{ form_errors(current_internship_form.preferredCountries) }}&lt;/span&gt;
  &lt;/div&gt;
  &lt;div class="form-group col-xs-12"&gt;
      {{ form_label(current_internship_form.internshipProgram, 'What type of training agreement will you have?', { 'label_attr': {'class': 'label-text'} }) }}
      {{ form_widget(current_internship_form.internshipProgram, { 'id': 'internship_program', 'attr': {'class': 'form-control '}}) }}
      &lt;span class="error text-danger small"&gt;{{ form_errors(current_internship_form.internshipProgram) }}&lt;/span&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Is there a mistake in my code or is it somehow related to multiple choice selection? Has anyone had similar issue and know how to solve it?</p>