fix(kubernetes): tolerate an empty manifest in KubernetesDeployManifestOperation (#6337)

in case the result of a bake stage is empty.  Without this there's:

java.lang.NullPointerException
	at com.netflix.spinnaker.clouddriver.kubernetes.op.manifest.KubernetesDeployManifestOperation.getManifestsFromDescription(KubernetesDeployManifestOperation.java:234)
	at com.netflix.spinnaker.clouddriver.kubernetes.op.manifest.KubernetesDeployManifestOperation.operate(KubernetesDeployManifestOperation.java:75)
	at com.netflix.spinnaker.clouddriver.kubernetes.op.manifest.KubernetesDeployManifestOperation.operate(KubernetesDeployManifestOperation.java:48)

because KubernetesDeployManifestDescription.getManifestsFromDescription assumes that description.getManifest() never returns null.

Author: dbyron-sf

clouddriver-kubernetes/src/main/java/com/netflix/spinnaker/clouddriver/kubernetes/op/manifest/KubernetesDeployManifestOperation.java

@@ -228,12 +228,16 @@ private List<KubernetesManifest> getManifestsFromDescription() {
       // check `manifest` for backwards compatibility until all existing stages have been updated.
       @SuppressWarnings("deprecation")
       KubernetesManifest manifest = description.getManifest();
-      log.warn(
-          "Relying on deprecated single manifest input (account: {}, kind: {}, name: {})",
-          accountName,
-          manifest.getKind(),
-          manifest.getName());
-      inputManifests = ImmutableList.of(manifest);
+
+      // manifest may be null as well, so check
+      if (manifest != null) {
+        log.warn(
+            "Relying on deprecated single manifest input (account: {}, kind: {}, name: {})",
+            accountName,
+            manifest.getKind(),
+            manifest.getName());
+        inputManifests = ImmutableList.of(manifest);
+      }
     }
     inputManifests = inputManifests.stream().filter(Objects::nonNull).collect(Collectors.toList());
     return inputManifests;

clouddriver-kubernetes/src/test/java/com/netflix/spinnaker/clouddriver/kubernetes/op/KubernetesDeployManifestOperationTest.java

@@ -503,6 +503,13 @@ void replaceStrategyWithLabelSelector() {
             anyString());
   }
 
+  @Test
+  void deployEmptyResource() {
+    KubernetesDeployManifestDescription deployManifestDescription =
+        baseDeployDescription("deploy/empty-resource.yml");
+    deploy(deployManifestDescription);
+  }
+
   private static KubernetesDeployManifestDescription baseDeployDescription(String manifest) {
     return baseDeployDescription(manifest, false);
   }

clouddriver-kubernetes/src/test/resources/com/netflix/spinnaker/clouddriver/kubernetes/op/deploy/empty-resource.yml

@@ -0,0 +1 @@
+