title: Django and empty formset are valid
body: <p>I have a little problem with the formset.</p>
  
  <p>I must display several formsets in a page, and each formset has several forms.
  So i did something like that :</p>
  
  <pre><code>#GET
  for prod in products:
       ProductFormSet = modelformset_factory(Product,exclude=('date',),extra=prod.amount)
       formsset.append(ProductFormSet(prefix="prod_%d"%prod.pk))
  
  #POST
  for prod in products:
       ProductFormSet = modelformset_factory(Product,exclude=('date',),extra=prod.amount)
       formsset.append(ProductFormSet(request.POST,prefix="prod_%d"%prod.pk))
  </code></pre>
  
  <p>The problem is when I submit the page, the empties forms are 'automatically' valid (without check),
  but if I fill one field in one form, the check works on it.</p>
  
  <p>I don't know why, so if anyone has an idea,</p>
  
  <p>thanks.</p>