title: django-taggit: make the tags not required in the admin
body: <p>I've started using django-taggit and it seems to fit the bill. But for me there is still an issue with the admin site:  </p>
  
  <p>I included the <code>tags</code> attribute in the ModelAdmin like this:</p>
  
  <pre><code>class MyModel(db.models.Model):
      name = models.CharField(max_length=200)
      tags = TaggableManager()
  
  class MyModelAdmin(admin.ModelAdmin):
      fieldsets = (
          (None, {
              'fields': ('name', 'tags')
          }),
      )
  </code></pre>
  
  <p>And everything goes as expected. But when I edit a model in the admin, I get an error, if the TagField is empty. The form seems to be happy with just a blank, and that results in no tags being saved (as expected). But an empty tag field triggers the error.</p>
  
  <p>What can I do?</p>