Fix handling of non-existing repos during registry phase 2 migration

Context

Related to #356984 (closed).

This is a fix for a bug introduced in !83746 (merged). Please read that MR description for additional context. Back then we forgot to ensure that the migration_import_done_at attribute on the ContainerRepository object is set when the import is marked as done. This MR fixes that.

Note that all of this migration logic is gated behind a feature flag that is not enabled on gitlab.com.

What does this MR do and why?

Ensures that the migration_import_done_at attribute on the ContainerRepository object is always set when the migration_status transitions to import_done.

How to set up and validate locally

1. Have a GDK ready with the registry enabled.

2. Set up the feature flags:

   Feature.enable(:container_registry_migration_phase2_enabled)

We are ready to test things.

1. Create a dummy image:

   repo = FactoryBot.create(:container_repository, project: Project.first)

2. Getting a registry set up with all the parts for the migration is quite involved. Instead we're going to "stub" the registry response from the pre import call. Replace ContainerRepository#migration_pre_import with:

   def migration_pre_import
      :already_imported
   end

3. Comment out the ContainerRepository#start_pre_import function (L262).

4. Reload the console if necessary:

   reload!

5. Start the pre-import:

   repo.start_pre_import

6. Inspect the repo:

   [7] pry(main)> repo
   => #<ContainerRepository:0x00007f9d26b476a0
   id: 10,
   project_id: 88,
   name: "test_image_1",
   created_at: Thu, 31 Mar 2022 13:38:43.340949000 UTC +00:00,
   updated_at: Thu, 31 Mar 2022 13:38:47.594355000 UTC +00:00,
   status: nil,
   expiration_policy_started_at: nil,
   expiration_policy_cleanup_status: "cleanup_unscheduled",
   expiration_policy_completed_at: nil,
   migration_pre_import_started_at: Thu, 31 Mar 2022 13:38:47.583746000 UTC +00:00,
   migration_pre_import_done_at: nil,
   migration_import_started_at: nil,
   migration_import_done_at: Thu, 31 Mar 2022 13:38:47.591828000 UTC +00:00,
   migration_aborted_at: nil,
   migration_skipped_at: nil,
   migration_retries_count: 0,
   migration_skipped_reason: "native_import",
   migration_state: "import_done",
   migration_aborted_in_state: nil,
   migration_plan: nil>

Note that migration_import_done_at is set.

MR acceptance checklist

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