Update stable checks to consider no downstream pipeline is generated

The package-and-test bridge can be canceled which interrupts the
generation of the downstream pipeline. This commit updates Danger stable
branch checks to account for these scenarios. This will avoid Danger
code failures when no downstream pipeline can be found, e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/3943016314.

spec/tooling/danger/stable_branch_spec.rb

@@ -177,20 +177,37 @@
         it_behaves_like 'bypassing when flaky test or docs only'
       end
 
-      context 'when no package-and-test job is found' do
+      context 'when no package-and-test bridge is found' do
         let(:pipeline_bridges_response) { nil }
 
         it_behaves_like 'with a failure', described_class::NEEDS_PACKAGE_AND_TEST_MESSAGE
         it_behaves_like 'bypassing when flaky test or docs only'
       end
 
-      context 'when package-and-test job is being created' do
+      context 'when package-and-test bridge is created' do
         let(:pipeline_bridge_state) { 'created' }
 
         it_behaves_like 'with a warning', described_class::WARN_PACKAGE_AND_TEST_MESSAGE
         it_behaves_like 'bypassing when flaky test or docs only'
       end
 
+      context 'when package-and-test bridge has been canceled and no downstream pipeline is generated' do
+        let(:pipeline_bridge_state) { 'canceled' }
+
+        let(:pipeline_bridges_response) do
+          [
+            {
+              'name' => 'e2e:package-and-test',
+              'status' => pipeline_bridge_state,
+              'downstream_pipeline' => nil
+            }
+          ]
+        end
+
+        it_behaves_like 'with a failure', described_class::NEEDS_PACKAGE_AND_TEST_MESSAGE
+        it_behaves_like 'bypassing when flaky test or docs only'
+      end
+
       context 'when package-and-test job is in a non-successful state' do
         let(:package_and_qa_state) { 'running' }
 

tooling/danger/stable_branch.rb

@@ -102,7 +102,7 @@ def package_and_test_bridge_and_pipeline_status
         if bridge['status'] == 'created'
           bridge['status']
         else
-          bridge.fetch('downstream_pipeline').fetch('status')
+          bridge.fetch('downstream_pipeline')&.fetch('status')
         end
       end