Don't output empty sections in the manifests

In our kube-dns manifest for 1.6 we often had an empty section,
normalization converted this to `{}` which causes `kubectl apply` to
fail.

We can simply skip empty objects when outputing.

pkg/assets/builder.go

@@ -111,6 +111,11 @@ func (a *AssetBuilder) RemapManifest(data []byte) ([]byte, error) {
 			return nil, fmt.Errorf("error remapping images: %v", err)
 		}
 
+		// Don't serialize empty objects - they confuse yaml parsers
+		if manifest.IsEmptyObject() {
+			continue
+		}
+
 		y, err := manifest.ToYAML()
 		if err != nil {
 			return nil, fmt.Errorf("error re-marshaling manifest: %v", err)

pkg/kubemanifest/manifest.go

@@ -65,3 +65,8 @@ func (m *Manifest) accept(visitor Visitor) error {
 	})
 	return err
 }
+
+// IsEmptyObject checks if the object has no keys set (i.e. `== {}`)
+func (m *Manifest) IsEmptyObject() bool {
+	return len(m.data) == 0
+}