[Broker] Fix NPE when subscription is already removed (apache#14363)

* [Broker] Fix NPE when subscription is already removed

* Cover same case for NonPersistentTopic

Master Issue: apache#14362

### Motivation

There is current a race condition when we remove a subscription. The race and how to reproduce it is described in the apache#14362. One of the consequences of the race is that there is a chance we try to remove the subscription from the topic twice. This leads to an NPE, as described in the issue.

### Modifications

* Verify that the `sub` is not null before getting its stats.

### Verifying this change

This is a trivial change.

(cherry picked from commit aee1e7d)

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/nonpersistent/NonPersistentTopic.java

@@ -1063,10 +1063,12 @@ public CompletableFuture<Void> unsubscribe(String subscriptionName) {
         // That creates deadlock. so, execute remove it in different thread.
         return CompletableFuture.runAsync(() -> {
             NonPersistentSubscription sub = subscriptions.remove(subscriptionName);
-            // preserve accumulative stats form removed subscription
-            SubscriptionStatsImpl stats = sub.getStats();
-            bytesOutFromRemovedSubscriptions.add(stats.bytesOutCounter);
-            msgOutFromRemovedSubscriptions.add(stats.msgOutCounter);
+            if (sub != null) {
+                // preserve accumulative stats form removed subscription
+                SubscriptionStatsImpl stats = sub.getStats();
+                bytesOutFromRemovedSubscriptions.add(stats.bytesOutCounter);
+                msgOutFromRemovedSubscriptions.add(stats.msgOutCounter);
+            }
         }, brokerService.executor());
     }
 

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

@@ -1050,10 +1050,12 @@ public void deleteCursorFailed(ManagedLedgerException exception, Object ctx) {
 
     void removeSubscription(String subscriptionName) {
         PersistentSubscription sub = subscriptions.remove(subscriptionName);
-        // preserve accumulative stats form removed subscription
-        SubscriptionStatsImpl stats = sub.getStats(false, false, false);
-        bytesOutFromRemovedSubscriptions.add(stats.bytesOutCounter);
-        msgOutFromRemovedSubscriptions.add(stats.msgOutCounter);
+        if (sub != null) {
+            // preserve accumulative stats form removed subscription
+            SubscriptionStatsImpl stats = sub.getStats(false, false, false);
+            bytesOutFromRemovedSubscriptions.add(stats.bytesOutCounter);
+            msgOutFromRemovedSubscriptions.add(stats.msgOutCounter);
+        }
     }
 
     /**