Fix ZK LeaderInitiator Checkstyle violation

**Cherry-pick to 5.0.x**

spring-integration-zookeeper/src/main/java/org/springframework/integration/zookeeper/leader/LeaderInitiator.java

@@ -221,7 +221,7 @@ public class LeaderInitiator implements SmartLifecycle {
 	 */
 	public Context getContext() {
 		if (this.leaderSelector == null) {
-			return nullContext;
+			return this.nullContext;
 		}
 		return this.context;
 	}