NPE when creating consumer stats for subscription __compaction from checkAndUnblockIfStuck()

[devinbost]

@rdhabalia I believe this is a bug with #9789

When running PersistentSubscription.checkAndUnblockIfStuck, it complains on the internal __compaction topic.
It occurred immediately after starting the brokers and is repeating the exception (filling the logs).

2021-04-29T00:11:51,161 [pulsar-stats-updater-29-1] ERROR org.apache.pulsar.broker.service.persistent.PersistentTopic - Got exception when creating consumer stats for subscription __compaction: null
java.lang.NullPointerException: null
at org.apache.pulsar.broker.service.persistent.PersistentSubscription.checkAndUnblockIfStuck(PersistentSubscription.java:1101) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.persistent.PersistentTopic.lambda$updateRates$45(PersistentTopic.java:1642) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap$Section.forEach(ConcurrentOpenHashMap.java:387) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap.forEach(ConcurrentOpenHashMap.java:159) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.persistent.PersistentTopic.updateRates(PersistentTopic.java:1585) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.PulsarStats.lambda$null$1(PulsarStats.java:132) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap$Section.forEach(ConcurrentOpenHashMap.java:387) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap.forEach(ConcurrentOpenHashMap.java:159) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.PulsarStats.lambda$null$3(PulsarStats.java:129) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap$Section.forEach(ConcurrentOpenHashMap.java:387) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap.forEach(ConcurrentOpenHashMap.java:159) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.PulsarStats.lambda$updateStats$4(PulsarStats.java:118) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap$Section.forEach(ConcurrentOpenHashMap.java:387) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap.forEach(ConcurrentOpenHashMap.java:159) ~[org.apache.pulsar-pulsar-common-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.PulsarStats.updateStats(PulsarStats.java:108) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.pulsar.broker.service.BrokerService.updateRates(BrokerService.java:1500) ~[org.apache.pulsar-pulsar-broker-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.bookkeeper.mledger.util.SafeRun$1.safeRun(SafeRun.java:32) [org.apache.pulsar-managed-ledger-2.8.0-SNAPSHOT.jar:2.8.0-SNAPSHOT]
at org.apache.bookkeeper.common.util.SafeRunnable.run(SafeRunnable.java:36) [org.apache.bookkeeper-bookkeeper-common-4.13.0.jar:4.13.0]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_282]
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) [?:1.8.0_282]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) [?:1.8.0_282]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) [?:1.8.0_282]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_282]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_282]
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [io.netty-netty-common-4.1.63.Final.jar:4.1.63.Final]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_282]