this doesn't handle the possibility of `parts` being `None`:


```
Error in migration: feedback:0012_fix_firefoxos_version
Traceback (most recent call last):
  File "./manage.py", line 27, in <module>
    manage.main()
  File "/Users/rlr/dev/fjord/vendor/src/funfactory/funfactory/manage.py", line 143, in main
    execute_manager(current_settings)
  File "/Users/rlr/dev/fjord/vendor/lib/python/django/core/management/__init__.py", line 459, in execute_manager
    utility.execute()
  File "/Users/rlr/dev/fjord/vendor/lib/python/django/core/management/__init__.py", line 382, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/rlr/dev/fjord/vendor/lib/python/django/core/management/base.py", line 196, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/Users/rlr/dev/fjord/vendor/lib/python/django/core/management/base.py", line 232, in execute
    output = self.handle(*args, **options)
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/management/commands/migrate.py", line 107, in handle
    ignore_ghosts = ignore_ghosts,
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/__init__.py", line 219, in migrate_app
    success = migrator.migrate_many(target, workplan, database)
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/migrators.py", line 235, in migrate_many
    result = migrator.__class__.migrate_many(migrator, target, migrations, database)
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/migrators.py", line 310, in migrate_many
    result = self.migrate(migration, database)
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/migrators.py", line 133, in migrate
    result = self.run(migration)
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/migrators.py", line 107, in run
    return self.run_migration(migration)
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/migrators.py", line 81, in run_migration
    migration_function()
  File "/Users/rlr/dev/fjord/vendor-local/packages/South/south/migration/migrators.py", line 57, in <lambda>
    return (lambda: direction(orm))
  File "/Users/rlr/dev/fjord/fjord/feedback/migrations/0012_fix_firefoxos_version.py", line 47, in forwards
    for part in parts:
TypeError: 'NoneType' object is not iterable
```