title: ManyToMany in Django admin: select none
body: <p>Having <code>A = ManyToManyField(B, null=True, blank=True)</code>, when I go in A's admin page, it seems I can't unselect every entries in the ManyToMany box after having clicked on a B element.<br>
  And even if I don't click on any entry, there is a related B element selected after saving (the first B element I guess).</p>
  
  <p>But I want to add A elements without having to relate them to any one of B...</p>
  
  <p>Is there any way to say to Django admin to select no element? (other than creating a dummy B element for those situations)</p>