Fix migration when project repository is missing

The data migration looks for code owner file and errs
if repository is missing.

app/models/repository.rb

@@ -525,6 +525,8 @@ class Repository
 
   # items is an Array like: [[oid, path], [oid1, path1]]
   def blobs_at(items)
+    return [] unless exists?
+
     raw_repository.batch_blobs(items).map { |blob| Blob.decorate(blob, project) }
   end
 

ee/changelogs/unreleased/1979-blobs-at.yml

+---
+title: Fix background migration error when project repository is missing
+merge_request: 9392
+author:
+type: fixed

ee/spec/lib/gitlab/background_migration/migrate_approver_to_approval_rules_spec.rb

@@ -254,5 +254,17 @@ describe Gitlab::BackgroundMigration::MigrateApproverToApprovalRules do
       end.not_to raise_error
     end
   end
+
+  context 'when project has no repository' do
+    let(:project_without_repository) { create(:project) }
+    let(:target) { create(:merge_request, target_project: project_without_repository, source_project: project_without_repository) }
+    let(:target_type) { 'MergeRequest' }
+
+    it "does not err" do
+      expect do
+        described_class.new.perform(target_type, target.id)
+      end.not_to raise_error
+    end
+  end
 end
 # rubocop:enable RSpec/FactoriesInMigrationSpecs

spec/models/repository_spec.rb

@@ -1237,6 +1237,27 @@ describe Repository do
     end
   end
 
+  describe '#blobs_at' do
+    let(:empty_repository) { create(:project_empty_repo).repository }
+
+    it 'returns empty array for an empty repository' do
+      # rubocop:disable Style/WordArray
+      expect(empty_repository.blobs_at(['master', 'foobar'])).to eq([])
+      # rubocop:enable Style/WordArray
+    end
+
+    it 'returns blob array for a non-empty repository' do
+      repository.create_file(User.last, 'foobar', 'CONTENT', message: 'message', branch_name: 'master')
+
+      # rubocop:disable Style/WordArray
+      blobs = repository.blobs_at([['master', 'foobar']])
+      # rubocop:enable Style/WordArray
+
+      expect(blobs.first.name).to eq('foobar')
+      expect(blobs.size).to eq(1)
+    end
+  end
+
   describe '#root_ref' do
     it 'returns a branch name' do
       expect(repository.root_ref).to be_an_instance_of(String)