title: Django South migration not working with null = True and blank = True
body: <p>I am using <a href="http://south.aeracode.org/" rel="noreferrer">South</a> with Django for database migrations.</p>
  
  <p>In my <code>models.py</code> I changed one of the fields from</p>
  
  <pre><code>class User(models.Model):
      group = models.ForeignKey(Group)
  </code></pre>
  
  <p>to</p>
  
  <pre><code>class User(models.Model):
      group = models.ForeignKey(Group, null = True, blank = True)
  </code></pre>
  
  <p>In other words, I wanted to make the <code>group</code> field for <code>User</code> optional.</p>
  
  <p>Then when I tried to run a schemamigration, South gave me this error</p>
  
  <pre><code>(doors)hobbes3@hobbes3:~/Sites/mysite$ python manage.py schemamigration doors --auto
   ? The field 'User.group' does not have a default specified, yet is NOT NULL.
   ? Since you are making this field nullable, you MUST specify a default
   ? value to use for existing rows. Would you like to:
   ?  1. Quit now, and add a default to the field in models.py
   ?  2. Specify a one-off value to use for existing columns now
   ?  3. Disable the backwards migration by raising an exception.
   ? Please select a choice: 
  </code></pre>
  
  <p>Why is South complaining? Didn't I specify a default of NULL with <code>null = True</code> and <code>blank = True</code>?</p>
  
  <p>In case it matters this is what my current table looks like for <code>doors_user</code></p>
  
  <pre><code>mysite=# \d doors_user
                                      Table "public.doors_user"
     Column    |           Type           |                        Modifiers                        
  -------------+--------------------------+---------------------------------------------------------
   id          | integer                  | not null default nextval('doors_user_id_seq'::regclass)
   group_id    | integer                  | not null
   user_type   | character varying(1)     | not null default 't'::character varying
   comment     | text                     | not null
   email       | character varying(75)    | not null
   password    | character varying(135)   | not null
   first_name  | character varying(135)   | not null
   last_name   | character varying(135)   | not null
   phone       | character varying(135)   | not null
   status      | character varying(1)     | not null default 'p'::character varying
   location_id | integer                  | 
   t_created   | timestamp with time zone | not null
   t_modified  | timestamp with time zone | not null
  Indexes:
      "doors_user_pkey" PRIMARY KEY, btree (id)
      "doors_user_group_id" btree (group_id)
      "doors_user_location_id" btree (location_id)
  Foreign-key constraints:
      "group_id_refs_id_2fde5e861cc0e5fe" FOREIGN KEY (group_id) REFERENCES doors_group(id) DEFERRABLE INITIALLY DEFERRED
      "location_id_refs_id_13c85dcc5cba5e23" FOREIGN KEY (location_id) REFERENCES doors_location(id) DEFERRABLE INITIALLY DEFERRED
  Referenced by:
      TABLE "doors_property" CONSTRAINT "owner_id_refs_id_7a3a10af3eba8739" FOREIGN KEY (owner_id) REFERENCES doors_user(id) DEFERRABLE INITIALLY DEFERRED
      TABLE "doors_order" CONSTRAINT "user_action_id_refs_id_79506d7c5228f713" FOREIGN KEY (user_action_id) REFERENCES doors_user(id) DEFERRABLE INITIALLY DEFERRED
      TABLE "doors_order" CONSTRAINT "user_created_id_refs_id_79506d7c5228f713" FOREIGN KEY (user_created_id) REFERENCES doors_user(id) DEFERRABLE INITIALLY DEFERRED
      TABLE "doors_log" CONSTRAINT "user_id_refs_id_3ce582a126688737" FOREIGN KEY (user_id) REFERENCES doors_user(id) DEFERRABLE INITIALLY DEFERRED
      TABLE "doors_ordercomment" CONSTRAINT "user_id_refs_id_6d10d6e79572e14d" FOREIGN KEY (user_id) REFERENCES doors_user(id) DEFERRABLE INITIALLY DEFERRED
  </code></pre>
  
  <p>And the <code>SELECT</code> statement</p>
  
  <pre><code>mysite=# select * from doors_user;
   id | group_id | user_type | comment |      email       | password | first_name | last_name | phone | status | location_id |          t_created           |          t_modified           
  ----+----------+-----------+---------+------------------+----------+------------+-----------+-------+--------+-------------+------------------------------+-------------------------------
    1 |        1 | w         |         | blah12@gmail.com | ads      | Michael    | Anderson  |       | a      |             | 2012-03-04 06:44:44.97263-05 | 2012-03-04 06:44:44.972661-05
  (1 row)
  </code></pre>