Fix the wrong parameter in the log. (apache#14309)

### Motivation

Bad parameters in the log will always print "null", which can confuse users.

### Modifications

- Correct exception parameter.

(cherry picked from commit 9b003f9)

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/SystemTopicBasedTopicPoliciesService.java

@@ -316,7 +316,7 @@ private void initPolicesCache(SystemTopicClient.Reader<PulsarEvent> reader, Comp
                 reader.readNextAsync().whenComplete((msg, e) -> {
                     if (e != null) {
                         log.error("[{}] Failed to read event from the system topic.",
-                                reader.getSystemTopic().getTopicName(), ex);
+                                reader.getSystemTopic().getTopicName(), e);
                         future.completeExceptionally(e);
                         readerCaches.remove(reader.getSystemTopic().getTopicName().getNamespaceObject());
                         policyCacheInitMap.remove(reader.getSystemTopic().getTopicName().getNamespaceObject());