title: Why is django admin not accepting Nullable foreign keys?
body: <p>Here is a simplified version of one of my models:</p>
  
  <pre><code>class ImportRule(models.Model):
    feed = models.ForeignKey(Feed)
    name = models.CharField(max_length=255)
    feed_provider_category = models.ForeignKey(FeedProviderCategory, null=True)
    target_subcategories = models.ManyToManyField(Subcategory)
  </code></pre>
  
  <p>This class manages a rule for importing a list of items from a feed into the database.</p>
  
  <p>The admin system won't let me add an ImportRule without selecting a feed_provider_category despite it being declared in the model as nullable. The database (SQLite at the moment) even checks out ok:</p>
  
  <pre><code>&gt;&gt;&gt; .schema
  ...
  CREATE TABLE "someapp_importrule" (
    "id" integer NOT NULL PRIMARY KEY,
    "feed_id" integer NOT NULL REFERENCES "someapp_feed" ("id"),
    "name" varchar(255) NOT NULL,
    "feed_provider_category_id" integer REFERENCES "someapp_feedprovidercategory" ("id"),
  );
  ...
  </code></pre>
  
  <p>I can create the object in the python shell easily enough:</p>
  
  <pre><code>f = Feed.objects.get(pk=1)
  i = ImportRule(name='test', feed=f)
  i.save()
  </code></pre>
  
  <p>...but the admin system won't let me edit it, of course.
  How can I get the admin to let me edit/create objects without specifying that foreign key?</p>