Repository#blob_at should return nil for 00000000... sha

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/repository.rb

@@ -173,8 +173,10 @@ class Repository
   end
 
   def blob_at(sha, path)
+    unless Gitlab::Git.blank_ref?(sha)
       Gitlab::Git::Blob.find(self, sha, path)
     end
+  end
 
   def blob_by_oid(oid)
     Gitlab::Git::Blob.raw(self, oid)

spec/models/repository_spec.rb

@@ -25,4 +25,12 @@ describe Repository do
 
     it { is_expected.to eq('c1acaa58bbcbc3eafe538cb8274ba387047b69f8') }
   end
+
+  describe :blob_at do
+    context 'blank sha' do
+      subject { repository.blob_at(Gitlab::Git::BLANK_SHA, '.gitignore') }
+
+      it { is_expected.to be_nil }
+    end
+  end
 end