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

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)