Migration of BMC power credentials fails with manual driver

$ bin/database --preserve run -- bin/maas-region dbupgrade
Operations to perform:
  Apply all migrations: auth, contenttypes, maasserver, metadataserver, piston3, sessions, sites
Running migrations:
  Applying maasserver.0288_rootkey_material_secret... OK
  Applying maasserver.0289_vault_secret... OK
  Applying maasserver.0290_migrate_node_power_parameters...Traceback (most recent call last):
  File "/home/ubuntu/maas/bin/maas-region", line 33, in <module>
    sys.exit(load_entry_point('maas', 'console_scripts', 'maas-region')())
  File "/home/ubuntu/maas/src/maasserver/region_script.py", line 78, in run
    run_django(is_snap, is_devenv)
  File "/home/ubuntu/maas/src/maasserver/region_script.py", line 67, in run_django
    management.execute_from_command_line()
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 354, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "/home/ubuntu/maas/src/maasserver/management/commands/dbupgrade.py", line 125, in handle
    call_command(
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 181, in call_command
    return command.execute(*args, **defaults)
  File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "/usr/lib/python3/dist-packages/django/core/management/base.py", line 89, in wrapped
    res = handle_func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/core/management/commands/migrate.py", line 244, in handle
    post_migrate_state = executor.migrate(
  File "/usr/lib/python3/dist-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/usr/lib/python3/dist-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/usr/lib/python3/dist-packages/django/db/migrations/executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/lib/python3/dist-packages/django/db/migrations/migration.py", line 126, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/lib/python3/dist-packages/django/db/migrations/operations/special.py", line 190, in database_forwards
    self.code(from_state.apps, schema_editor)
  File "/home/ubuntu/maas/src/maasserver/migrations/maasserver/0290_migrate_node_power_parameters.py", line 49, in move_secrets
    for name, value in power_parameters.items():
AttributeError: 'str' object has no attribute 'items'

By adding a pdb, we can see

> /home/ubuntu/maas/src/maasserver/migrations/maasserver/0290_migrate_node_power_parameters.py(52)move_secrets()
-> for name, value in power_parameters.items():
(Pdb) power_parameters
''
(Pdb) bmc_id
6
(Pdb) power_type
'manual'
(Pdb) bmc_id
6

that for manual power driver, power_parameters is an empty string, and cannot be iterated over