Fix NPE in checkSubscriptionTypesEnable (apache#12961)

### Motivation

The NPE may appear in `PersistentTopic.checkSubscriptionTypesEnable`.   The `topicPolicies.getSubscriptionTypesEnabled()` may be null.

The problem occurred when upgrading from pulsar version 2.7 to 2.8. We added `SubscriptionTypesEnable` in 2.8: apache#9401. When we initialize a topic's topicPolicy in pulasr 2.7, and then get the topic's topicPolicy in version 2.8, the `SubscriptionTypesEnable` will be null. This leads to the problem.

Here are steps to reproduce:
* Start broker 2.7
* Create a topic and init the topic policy
* Upgrade broker to 2.8
* We will get the `SubscriptionTypesEnable` with the value of null
```sh
➜  pulsar-281 bin/pulsar-admin topics get-subscription-types-enabled  my-topic
null
```
* When we consume from that topic, the issue occurs
```
Caused by: java.lang.NullPointerException
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.checkSubscriptionTypesEnable(PersistentTopic.java:3206) ~[io.streamnative-pulsar-broker-2.8.1.21.jar:2.8.1.21]
	at org.apache.pulsar.broker.service.persistent.PersistentTopic.subscribe(PersistentTopic.java:688) ~[io.streamnative-pulsar-broker-2.8.1.21.jar:2.8.1.21]
	at org.apache.pulsar.broker.service.ServerCnx.lambda$null$13(ServerCnx.java:1029) ~[io.streamnative-pulsar-broker-2.8.1.21.jar:2.8.1.21]
	at java.util.concurrent.CompletableFuture.uniComposeStage(CompletableFuture.java:1106) ~[?:?]
	... 31 more
```

### Modifications

* Use `CollectionUtils.isEmpty` to determine if the list is empty to fix the problem of throwing NPE.

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentTopic.java

@@ -73,6 +73,7 @@
 import org.apache.bookkeeper.mledger.impl.ManagedLedgerImpl;
 import org.apache.bookkeeper.mledger.impl.PositionImpl;
 import org.apache.bookkeeper.net.BookieId;
+import org.apache.commons.collections.CollectionUtils;
 import org.apache.commons.collections4.MapUtils;
 import org.apache.commons.lang3.StringUtils;
 import org.apache.pulsar.broker.PulsarServerException;
@@ -3232,7 +3233,7 @@ public boolean checkSubscriptionTypesEnable(SubType subType) throws Exception {
                 if (topicPolicies == null) {
                     return checkNsAndBrokerSubscriptionTypesEnable(topicName, subType);
                 } else {
-                    if (topicPolicies.getSubscriptionTypesEnabled().isEmpty()) {
+                    if (CollectionUtils.isEmpty(topicPolicies.getSubscriptionTypesEnabled())) {
                         return checkNsAndBrokerSubscriptionTypesEnable(topicName, subType);
                     }
                     return topicPolicies.getSubscriptionTypesEnabled().contains(subType);