fix nil dereference issue in fieldmeta

kyaml/fieldmeta/fieldmeta.go

@@ -17,7 +17,7 @@ import (
 type FieldMeta struct {
 	Schema spec.Schema
 
-	Extensions *XKustomize
+	Extensions XKustomize
 }
 
 type XKustomize struct {
@@ -62,9 +62,22 @@ func (fm *FieldMeta) Read(n *yaml.RNode) error {
 	return nil
 }
 
+func isExtensionEmpty(x XKustomize) bool {
+	if x.FieldSetter != nil {
+		return false
+	}
+	if x.SetBy != "" {
+		return false
+	}
+	if len(x.PartialFieldSetters) > 0 {
+		return false
+	}
+	return true
+}
+
 // Write writes the FieldMeta to a node
 func (fm *FieldMeta) Write(n *yaml.RNode) error {
-	if fm.Extensions != nil {
+	if !isExtensionEmpty(fm.Extensions) {
 		fm.Schema.VendorExtensible.AddExtension("x-kustomize", fm.Extensions)
 	} else {
 		delete(fm.Schema.VendorExtensible.Extensions, "x-kustomize")