GH-4027: Fix updateClusterId logic to expected == null

Fixed: #4027

Change condition from `!= null` to `== null` to only fetch `clusterId` when it's not set yet. 
This avoids unnecessary broker calls and improves performance while maintaining consistency with `clusterId()` method

This is a fix for an important omission after #2859

Signed-off-by: Choi Wang Gyu <[email protected]>

**Auto-cherry-pick to `3.3.x`**

Author: cwangg897

spring-kafka/src/main/java/org/springframework/kafka/core/KafkaAdmin.java

@@ -81,6 +81,7 @@
  * @author Valentina Armenise
  * @author Anders Swanson
  * @author Omer Celik
+ * @author Choi Wang Gyu
  *
  * @since 1.3
  */
@@ -300,7 +301,7 @@ public final boolean initialize() {
 	private void updateClusterId(Admin adminClient) throws InterruptedException, ExecutionException, TimeoutException {
 		try {
 			this.clusterIdLock.lock();
-			if (this.clusterId != null) {
+			if (this.clusterId == null) {
 				this.clusterId = adminClient.describeCluster().clusterId().get(this.operationTimeout,
 						TimeUnit.SECONDS);
 			}