[fix][broker] Fix NPE during active consumer is disconnected

[rdhabalia]

Motivation

Currently broker faces below NPE when active consumer disconnects and no other consumers are connected on fail-over subscription

07:11:28.357 [broker-topic-workers-OrderedExecutor-3-0] ERROR org.apache.bookkeeper.common.util.SingleThreadExecutor - Error while running task: Cannot invoke "org.apache.pulsar.broker.service
.Consumer.consumerName()" because "activeConsumer" is null
java.lang.NullPointerException: Cannot invoke "org.apache.pulsar.broker.service.Consumer.consumerName()" because "activeConsumer" is null
        at org.apache.pulsar.broker.service.persistent.PersistentDispatcherSingleActiveConsumer.readMoreEntries(PersistentDispatcherSingleActiveConsumer.java:353) ~[pulsar-broker-.jar:]
        at org.apache.pulsar.broker.service.persistent.PersistentDispatcherSingleActiveConsumer.lambda$dispatchEntriesToConsumer$2(PersistentDispatcherSingleActiveConsumer.java:242) ~[pulsar-broker-.jar:]
        at org.apache.bookkeeper.common.util.SingleThreadExecutor.safeRunTask(SingleThreadExecutor.java:137) [bookkeeper-common-4.17.1.jar:4.17.1]
        at org.apache.bookkeeper.common.util.SingleThreadExecutor.run(SingleThreadExecutor.java:113) [bookkeeper-common-4.17.1.jar:4.17.1]
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [netty-common-4.1.111.Final.jar:4.1.111.Final]
        at java.lang.Thread.run(Thread.java:840) [?:?]
        

Modifications

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Matching PR in forked repository

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.19%. Comparing base (bbc6224) to head (f5d90ef).
Report is 928 commits behind head on master.

Files with missing lines	Patch %	Lines
...tent/PersistentDispatcherSingleActiveConsumer.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #24008      +/-   ##
============================================
+ Coverage     73.57%   74.19%   +0.61%     
+ Complexity    32624    32267     -357     
============================================
  Files          1877     1854      -23     
  Lines        139502   143880    +4378     
  Branches      15299    16353    +1054     
============================================
+ Hits         102638   106746    +4108     
+ Misses        28908    28744     -164     
- Partials       7956     8390     +434     

Flag	Coverage Δ
inttests	26.75% <0.00%> (+2.16%)	⬆️
systests	23.26% <0.00%> (-1.07%)	⬇️
unittests	73.70% <50.00%> (+0.85%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...tent/PersistentDispatcherSingleActiveConsumer.java	67.75% <50.00%> (-1.18%)	⬇️

... and 1043 files with indirect coverage changes

[lhotari]

Not cherry-picking to branch-3.0 since it doesn't contain #22285 which introduced the NPE.