title: Symfony2 ManytoMany relation with nullable true
body: <p>I want to use an optional ManyToMany relation between Ordo_soins_perfusion and Ordo_soins_medicament entities. </p>
  
  <pre><code>  class Ordo_soins_perfusion
    {
  
   /**
   * @ORM\ManyToMany(targetEntity="Ordo_soins_medicament",cascade={"persist"})
    *@ORM\JoinTable(name="ordo_soinperf_soinmedoc",
   *      joinColumns={@ORM\JoinColumn(name="Ordo_soins_perfusion_id", referencedColumnName="id")},
   *      inverseJoinColumns={@ORM\JoinColumn(name="Ordo_soins_medicament_id", referencedColumnName="id",nullable=true)})
   */
  private $medoc;
  
      class Ordo_soins_medicament
      {
      /**
   * @var string
   *
   * @ORM\Column(name="medicament", type="string", length=255,nullable=true)
   */
  private $medicament;
  
  /**
   * @var string
   *
   * @ORM\Column(name="quantite", type="string", length=50,nullable=true)
   */
  private $quantite;
  </code></pre>
  
  <p>Now when i save a new Ordo_soins_perfusion object without filling the Ordo_soins_medicament form i found a new ligne created in the join table and in the Ordo_soins_medicament table.</p>
  
  <p>How the add a Ordo_soins_medicament object only if not null</p>
  
  <p>Thanks </p>