Set default value for description fields.

With field.null set, non-existing descriptions (all of them
after the upgrade) render as 'None' in the HTML. On SQLite,
South is anable to add the new column correctly with
field.empty, but setting the default like this works.

local_apps/pootle_misc/dbupdate.py

@@ -210,16 +210,18 @@ def update_tables_22000():
     logging.info("Updating existing database tables")
     from south.db import db
 
+    # For the sake of South bug 313, we set the default for these fields here:
+    # See http://south.aeracode.org/ticket/313
     from pootle_store.models import Suggestion
     table_name = Suggestion._meta.db_table
     field = Suggestion._meta.get_field('translator_comment_f')
-    field.null = True
+    field.default = u''
     db.add_column(table_name, field.name, field)
 
     from pootle_language.models import Language
     table_name = Language._meta.db_table
     field = Language._meta.get_field('description')
-    field.null = True
+    field.default = u''
     db.add_column(table_name, field.name, field)
 
     save_pootle_version(22000)