[fix][broker] Fix NPE when getting delayed delivery policy (apache#24512)

Author: codelipenghui

pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/PersistentTopicsBase.java

@@ -977,11 +977,13 @@ protected CompletableFuture<DelayedDeliveryPolicies> internalGetDelayedDeliveryP
                 TopicPolicies policies = op.orElseGet(TopicPolicies::new);
                 DelayedDeliveryPolicies delayedDeliveryPolicies = null;
                 if (policies.isDelayedDeliveryEnabledSet() && policies.isDelayedDeliveryTickTimeMillisSet()) {
-                    delayedDeliveryPolicies = DelayedDeliveryPolicies.builder()
+                    DelayedDeliveryPolicies.Builder builder = DelayedDeliveryPolicies.builder()
                             .tickTime(policies.getDelayedDeliveryTickTimeMillis())
-                            .active(policies.getDelayedDeliveryEnabled())
-                            .maxDeliveryDelayInMillis(policies.getDelayedDeliveryMaxDelayInMillis())
-                            .build();
+                            .active(policies.getDelayedDeliveryEnabled());
+                    if (policies.isDelayedDeliveryMaxDelayInMillisSet()) {
+                        builder.maxDeliveryDelayInMillis(policies.getDelayedDeliveryMaxDelayInMillis());
+                    }
+                    delayedDeliveryPolicies = builder.build();
                 }
                 if (delayedDeliveryPolicies == null && applied) {
                     delayedDeliveryPolicies = getNamespacePolicies(namespaceName).delayed_delivery_policies;

pulsar-common/src/main/java/org/apache/pulsar/common/policies/data/TopicPolicies.java

@@ -128,6 +128,10 @@ public boolean isDelayedDeliveryEnabledSet(){
         return delayedDeliveryEnabled != null;
     }
 
+    public boolean isDelayedDeliveryMaxDelayInMillisSet(){
+        return delayedDeliveryMaxDelayInMillis != null;
+    }
+
     public boolean isMaxUnackedMessagesOnSubscriptionSet() {
         return maxUnackedMessagesOnSubscription != null;
     }