resolver: Don't crash on nil objs (adobe#142)

If a YAML specifies an empty `initContainers:`, this will crash this
code if we also specify yamls. regardless of how pointless this is, this
is, it is valid k8s yaml that applies cleanly.

resolver/pkg/resolver.go

@@ -112,6 +112,9 @@ func (pt *imageTagTransformer) findAndReplaceTag(obj map[string]interface{}) err
 }
 
 func (pt *imageTagTransformer) updateContainers(obj map[string]interface{}, path string) error {
+	if obj[path] == nil {
+		return nil
+	}
 	containers := obj[path].([]interface{})
 	for i := range containers {
 		container := containers[i].(map[string]interface{})
@@ -134,6 +137,9 @@ func (pt *imageTagTransformer) updateContainers(obj map[string]interface{}, path
 }
 
 func (pt *imageTagTransformer) updateContainer(obj map[string]interface{}, path string) error {
+	if obj[path] == nil {
+		return nil
+	}
 	container := obj[path].(map[string]interface{})
 	image, found := container["image"]
 	if found {

resolver/pkg/resolver_test.go

@@ -40,6 +40,9 @@ func TestNoError(t *testing.T) {
 		{"zk", map[string]string{
 			"zk-image": "dummy",
 		}},
+		{"emptyinit", map[string]string{
+			"helloworld-image": "docker.io/kube/hello/image:tag",
+		}},
 	}
 	for _, testcase := range testcases {
 		t.Run(testcase.name, func(t *testing.T) {

resolver/pkg/testdata/emptyinit.expected.yaml

@@ -0,0 +1,14 @@
+apiVersion: apps/v1
+kind: CronWorkFlow
+metadata:
+  name: aaa
+  namespace: stats-dev
+spec:
+  workflowSpec:
+    metadata:
+      labels:
+        app: app
+    templates:
+      container:
+        image: docker.io/kube/hello/image:tag
+      initContainers: null

resolver/pkg/testdata/emptyinit.yaml

@@ -0,0 +1,14 @@
+apiVersion: apps/v1
+kind: CronWorkFlow
+metadata:
+  name: aaa
+  namespace: stats-dev
+spec:
+  workflowSpec:
+    metadata:
+      labels:
+        app: app
+    templates:
+      initContainers:
+      container:
+        image: helloworld-image