Add nullable annotation on ContainerProperties.transactionManager getter/setter

Author: jisoo.yoon

spring-kafka/src/main/java/org/springframework/kafka/listener/ContainerProperties.java

@@ -543,6 +543,7 @@ public Long getIdlePartitionEventInterval() {
 		return this.idlePartitionEventInterval;
 	}
 
+	@Nullable
 	public PlatformTransactionManager getTransactionManager() {
 		return this.transactionManager;
 	}
@@ -555,7 +556,7 @@ public PlatformTransactionManager getTransactionManager() {
 	 * @since 1.3
 	 * @see #setAckMode(AckMode)
 	 */
-	public void setTransactionManager(PlatformTransactionManager transactionManager) {
+	public void setTransactionManager(@Nullable PlatformTransactionManager transactionManager) {
 		this.transactionManager = transactionManager;
 	}