Fix null argument handling in background migration Rake task

See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84318

Changelog: fixed

lib/gitlab/database/migration_helpers.rb

@@ -961,7 +961,7 @@ module Gitlab
             "\n\n" \
             "Finalize it manualy by running" \
             "\n\n" \
-            "\tsudo gitlab-rake gitlab:background_migrations:finalize[#{job_class_name},#{table_name},#{column_name},'#{job_arguments.inspect.gsub(',', '\,')}']" \
+            "\tsudo gitlab-rake gitlab:background_migrations:finalize[#{job_class_name},#{table_name},#{column_name},'#{job_arguments.to_json.gsub(',', '\,')}']" \
             "\n\n" \
             "For more information, check the documentation" \
             "\n\n" \

lib/tasks/gitlab/background_migrations.rake

@@ -19,7 +19,7 @@ namespace :gitlab do
         args[:job_class_name],
         args[:table_name],
         args[:column_name],
-        Gitlab::Json.parse(args[:job_arguments]),
+        args[:job_arguments],
         connection: main_model.connection
       )
     end
@@ -38,7 +38,7 @@ namespace :gitlab do
             args[:job_class_name],
             args[:table_name],
             args[:column_name],
-            Gitlab::Json.parse(args[:job_arguments]),
+            args[:job_arguments],
             connection: model.connection
           )
         end

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -2211,7 +2211,7 @@ RSpec.describe Gitlab::Database::MigrationHelpers do
         job_class_name: 'CopyColumnUsingBackgroundMigrationJob',
         table_name: :events,
         column_name: :id,
-        job_arguments: [["id"], ["id_convert_to_bigint"]]
+        job_arguments: [["id"], ["id_convert_to_bigint"], nil]
       }
     end
@@ -2226,7 +2226,7 @@ RSpec.describe Gitlab::Database::MigrationHelpers do
             "\n\n" \
             "Finalize it manualy by running" \
             "\n\n" \
-            "\tsudo gitlab-rake gitlab:background_migrations:finalize[CopyColumnUsingBackgroundMigrationJob,events,id,'[[\"id\"]\\, [\"id_convert_to_bigint\"]]']" \
+            "\tsudo gitlab-rake gitlab:background_migrations:finalize[CopyColumnUsingBackgroundMigrationJob,events,id,'[[\"id\"]\\,[\"id_convert_to_bigint\"]\\,null]']" \
             "\n\n" \
             "For more information, check the documentation" \
             "\n\n" \

spec/tasks/gitlab/background_migrations_rake_spec.rb

@@ -42,6 +42,17 @@ RSpec.describe 'gitlab:background_migrations namespace rake tasks' do
       end
     end
+    context 'with a null parameter' do
+      let(:arguments) { %w[ProjectNamespaces::BackfillProjectNamespaces projects id] + ['[null\, "up"]'] }
+
+      it 'finalizes the matching migration' do
+        expect(Gitlab::Database::BackgroundMigration::BatchedMigrationRunner).to receive(:finalize)
+          .with('ProjectNamespaces::BackfillProjectNamespaces', 'projects', 'id', [nil, "up"], connection: connection)
+
+        expect { finalize_task }.to output(/Done/).to_stdout
+      end
+    end
+
     context 'when multiple database feature is enabled' do
       subject(:finalize_task) { run_rake_task("gitlab:background_migrations:finalize:#{ci_database_name}", *arguments) }