fix: Improve container registries migration script

[jopemachine]

Follow-up of #1917.

• Fixed a bug that occurs during downgrade() due to the absence of the extra column in the Image table.
• Modified the upgrade() to automatically generate the corresponding container registry row instead of raising an error when no container registry exists for the image canonical name.

example

🕙 18:58:36 ❯ ./py -m alembic upgrade +1
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 20218a73401b -> 1d42c726d8a3, Migrate container registry config storage from `Etcd` to `PostgreSQL`
INFO  [alembic.runtime.migration] Following container registry row auto-generated: "cr.backend.ai" with the project "multiarch".
INFO  [alembic.runtime.migration] If credential required for the auto-generated container registry rows, you should fill their credential columns manually.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version

[achimnol]

Still it fails:

  File "/home/joongi/bai-edge/src/ai/backend/manager/models/alembic/versions/1d42c726d8a3_migrate_container_registries_from_etcd_to_psql.py", line 457, in upgrade
    insert_registry_id_to_images_with_black_registry_id()
  File "/home/joongi/bai-edge/src/ai/backend/manager/models/alembic/versions/1d42c726d8a3_migrate_container_registries_from_etcd_to_psql.py", line 381, in insert_registry_id_to_images_with_black_registry_id
    registry_info = dict(registry_info)
                    ^^^^^^^^^^^^^^^^^^^
TypeError: 'NoneType' object is not iterable