Fix cluster apps artifact parsing for empty blob

This MR fixes cluster application artifact parsing for situations when
returned blob is empty. Empty blob was registered in production for
clusters without apps during uninstall. 'helm ls' also returns empty
string in this case even if output is set to json.

app/services/clusters/parse_cluster_applications_artifact_service.rb

@@ -46,6 +46,8 @@ module Clusters
       releases = []
 
       artifact.each_blob do |blob|
+        next if blob.empty?
+
         releases.concat(Gitlab::Kubernetes::Helm::Parsers::ListV2.new(blob).releases)
       end
 

spec/services/clusters/parse_cluster_applications_artifact_service_spec.rb

@@ -108,6 +108,17 @@ RSpec.describe Clusters::ParseClusterApplicationsArtifactService do
         end
       end
 
+      context 'blob is empty' do
+        let(:file) { fixture_file_upload(Rails.root.join("spec/fixtures/helm/helm_list_v2_empty_blob.json.gz")) }
+        let(:artifact) { create(:ci_job_artifact, :cluster_applications, job: job, file: file) }
+
+        it 'returns success' do
+          result = described_class.new(job, user).execute(artifact)
+
+          expect(result[:status]).to eq(:success)
+        end
+      end
+
       context 'job has deployment cluster' do
         context 'current user does not have access to deployment cluster' do
           let(:other_user) { create(:user) }