title: VichUploaderBundle : image_name can't be null error
body: <p>I'm using Symfony3.1 and I have just installed VichUploaderBundle.
  I have followed the documentation but I get this error means that <strong>Field 'image_name' can not be empty (null)</strong></p>
  
  <p><strong>SQLSTATE[23000]: Integrity constraint violation: 1048 Le champ 'image_name' ne peut être vide (null)</strong> </p>
  
  <p>Config</p>
  
  <pre><code>vich_uploader:
  db_driver: orm
  mappings:
      product_image:
          uri_prefix:         /images/actualites
          upload_destination: kernel.root_dir/../web/images/actualites
  
          inject_on_load:     false
          delete_on_update:   true
          delete_on_remove:   true
  </code></pre>
  
  <p>the entity</p>
  
  <pre><code>/**
   *
   * @Vich\UploadableField(mapping="actualite_image", fileNameProperty="imageName")
   *
   * @var File
   */
  private $imageFile;
  
  /**
   * @ORM\Column(type="string", length=255)
   *
   * @var string
   */
  private $imageName;
  
  /**
   * @ORM\Column(type="datetime")
   *
   * @var \DateTime
   */
  private $updatedAt;
  
  /**
   * @param File|\Symfony\Component\HttpFoundation\File\UploadedFile $image
   *
   * @return Actualite
   */
  
  public function setImageFile(File $image = null)
  {
      $this-&gt;imageFile = $image;
  
      if ($image) {
          // It is required that at least one field changes if you are using doctrine
          // otherwise the event listeners won't be called and the file is lost
          $this-&gt;updatedAt = new \DateTime('now');
      }
  
      return $this;
  }
  
  /**
   * @return File
   */
  public function getImageFile()
  {
      return $this-&gt;imageFile;
  }
  
  /**
   * @param string $imageName
   *
   * @return Actualite
   */
  public function setImageName($imageName)
  {
      $this-&gt;imageName = $imageName;
  
      return $this;
  }
  
  /**
   * @return string
   */
  public function getImageName()
  {
      return $this-&gt;imageName;
  }
  </code></pre>
  
  <p>FormType:</p>
  
  <pre><code>-&gt;add('imageFile', FileType::class)
  </code></pre>
  
  <p>twig</p>
  
  <pre><code>&lt;form class="form-horizontal" method="post" enctype="multipart/form-data"&gt;
   //...
   {{ form_widget(form.imageFile) }}
  </code></pre>
  
  <p>Action</p>
  
  <pre><code>public function creerAction(Request $request)
  {
      $entity = new Actualite();
      $form = $this-&gt;createForm(BaseType::class, $entity);
      $form-&gt;handleRequest($request);
  
      if ($form-&gt;isSubmitted() &amp;&amp; $form-&gt;isValid()) {
          $em = $this-&gt;getDoctrine()-&gt;getManager();
          $em-&gt;persist($entity);
          $em-&gt;flush();
  
          return $this-&gt;redirectToRoute('backend_actualite_liste');
      }
  
      return $this-&gt;render('Backend/Actualite/creer.html.twig',array(
              'form' =&gt; $form-&gt;createView(),
          )
      );
  }
  </code></pre>