Add missing initialized flag set to FlowJob

Issue #4142

Author: acktsap

spring-batch-core/src/main/java/org/springframework/batch/core/job/flow/FlowJob.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2019 the original author or authors.
+ * Copyright 2006-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,6 +35,7 @@
  *
  * @author Dave Syer
  * @author Mahmoud Ben Hassine
+ * @author Taeik Lim
  * @since 2.0
  */
 public class FlowJob extends AbstractJob {
@@ -84,6 +85,7 @@ public Step getStep(String stepName) {
 	 */
 	private void init() {
 		findSteps(flow, stepMap);
+		initialized = true;
 	}
 
 	/**