Set omitempty on DataSource for snapshot on PVCs

Add "omitempty" to spec.dataSource like it is done for all other
optional fields too.

Serialized code creates otherwise "null" fields and some introspection
tools rely on that value to determine if a value is optional or
required.

Kubernetes-commit: f866ca079a46fc97e584bef35363fd0f9036be73

core/v1/types.go

@@ -467,7 +467,7 @@ type PersistentVolumeClaimSpec struct {
 	// In the future, we plan to support more data source types and the behavior
 	// of the provisioner may change.
 	// +optional
-	DataSource *TypedLocalObjectReference `json:"dataSource" protobuf:"bytes,7,opt,name=dataSource"`
+	DataSource *TypedLocalObjectReference `json:"dataSource,omitempty" protobuf:"bytes,7,opt,name=dataSource"`
 }
 
 // PersistentVolumeClaimConditionType is a valid value of PersistentVolumeClaimCondition.Type