fix(manifest): Fix deployment of artifact only stages (#4044) (#4052)

Co-authored-by: Justin Field <justin.field@armory.io>
(cherry picked from commit d5614ea)

Co-authored-by: Gavin Bunney <409207+gavinbunney@users.noreply.github.com>

orca-clouddriver/src/main/groovy/com/netflix/spinnaker/orca/clouddriver/tasks/manifest/ResolveDeploySourceManifestTask.java

@@ -63,6 +63,10 @@ private ImmutableMap<String, Object> getOutputs(ManifestEvaluator.Result result)
   // Spel is not flattening nested List anymore
   // We flatten them because its possible have a List of manifests resolved by spel
   private List<Object> flattenNestedLists(Object manifests) {
+    if (manifests == null) {
+      return null;
+    }
+
     List<Object> result = new ArrayList<>();
     List<Object> tmp = (List<Object>) manifests;
 

orca-clouddriver/src/test/java/com/netflix/spinnaker/orca/clouddriver/tasks/manifest/ResolveDeploySourceManifestTaskTest.java

@@ -123,11 +123,31 @@ void manifestsWithListsIsFlattened() {
     assertThat(getManifests(result)).containsExactly(MANIFEST_1, MANIFEST_2);
   }
 
+  @Test
+  void stageWithoutManifestsHandled() {
+    ManifestEvaluator manifestEvaluator = mock(ManifestEvaluator.class);
+    ResolveDeploySourceManifestTask task = new ResolveDeploySourceManifestTask(manifestEvaluator);
+
+    StageExecutionImpl myStage = createStageWithManifests(null);
+
+    DeployManifestContext deployManifestContext = DeployManifestContext.builder().build();
+
+    when(manifestEvaluator.evaluate(any(), eq(deployManifestContext)))
+        .thenReturn(
+            new ManifestEvaluator.Result(
+                ImmutableList.of(MANIFEST_1), ImmutableList.of(), ImmutableList.of()));
+
+    TaskResult result = task.execute(myStage);
+    verify(manifestEvaluator, times(1)).evaluate(any(), eq(deployManifestContext));
+  }
+
   private StageExecutionImpl createStageWithManifests(ImmutableList<Object> manifestsByNamespace) {
     return new StageExecutionImpl(
         new PipelineExecutionImpl(ExecutionType.PIPELINE, "test"),
         "test",
-        new HashMap<>(ImmutableMap.of("manifests", manifestsByNamespace)));
+        manifestsByNamespace != null
+            ? new HashMap<>(ImmutableMap.of("manifests", manifestsByNamespace))
+            : new HashMap<>());
   }
 
   private static List<Map<Object, Object>> getManifests(TaskResult result) {