Broker startup fails when disk usage monitoring is disabled

[oleschoenburg]

Describe the bug

As reported by the community:

Starting a broker with disk usage monitoring disabled fails with an NPE while starting the command API.

To Reproduce

Set ZEEBE_BROKER_DATA_DISKUSAGEMONITORINGENABLED to false and start a broker

Expected behavior

Broker startup be successful, regardless of whether disk usage monitoring is enabled or not.

Log/Stacktrace

Full Stacktrace

2022-10-17 10:47:11.339 [Broker-0-Startup] [Broker-0-zb-actors-1] WARN 
      io.camunda.zeebe.broker.system - Aborting startup process due to exception during step Command API
java.lang.NullPointerException: Cannot invoke "io.camunda.zeebe.broker.system.monitoring.DiskSpaceUsageMonitor.addDiskUsageListener(io.camunda.zeebe.broker.system.monitoring.DiskSpaceUsageListener)" because the return value of "io.camunda.zeebe.broker.bootstrap.BrokerStartupContext.getDiskSpaceUsageMonitor()" is null
	at io.camunda.zeebe.broker.bootstrap.CommandApiServiceStep.lambda$startCommandApiService$2(CommandApiServiceStep.java:91) ~[zeebe-broker-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.broker.bootstrap.AbstractBrokerStartupStep.forwardExceptions(AbstractBrokerStartupStep.java:71) ~[zeebe-broker-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.broker.bootstrap.AbstractBrokerStartupStep.lambda$proceed$3(AbstractBrokerStartupStep.java:87) ~[zeebe-broker-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.future.FutureContinuationRunnable.run(FutureContinuationRunnable.java:28) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorJob.invoke(ActorJob.java:94) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorJob.execute(ActorJob.java:45) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorTask.execute(ActorTask.java:119) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorThread.executeCurrentTask(ActorThread.java:106) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorThread.doWork(ActorThread.java:87) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorThread.run(ActorThread.java:198) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
2022-10-17 10:47:11.355 [] [main] ERROR
      io.camunda.zeebe.broker.system - Failed to start broker 0!
java.util.concurrent.ExecutionException: Startup failed in the following steps: [Command API]. See suppressed exceptions for details.
	at io.camunda.zeebe.scheduler.future.CompletableActorFuture.get(CompletableActorFuture.java:142) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.future.CompletableActorFuture.get(CompletableActorFuture.java:109) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.FutureUtil.join(FutureUtil.java:21) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.future.CompletableActorFuture.join(CompletableActorFuture.java:198) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.broker.Broker.internalStart(Broker.java:101) ~[zeebe-broker-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.util.LogUtil.doWithMDC(LogUtil.java:23) ~[zeebe-util-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.broker.Broker.start(Broker.java:83) ~[zeebe-broker-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.broker.StandaloneBroker.run(StandaloneBroker.java:92) ~[camunda-zeebe-8.1.0.jar:8.1.0]
	at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:771) ~[spring-boot-2.7.4.jar:2.7.4]
	at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:755) ~[spring-boot-2.7.4.jar:2.7.4]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.7.4.jar:2.7.4]
	at io.camunda.zeebe.broker.StandaloneBroker.main(StandaloneBroker.java:82) ~[camunda-zeebe-8.1.0.jar:8.1.0]
Caused by: io.camunda.zeebe.scheduler.startup.StartupProcessException: Startup failed in the following steps: [Command API]. See suppressed exceptions for details.
	at io.camunda.zeebe.scheduler.startup.StartupProcess.aggregateExceptionsSynchronized(StartupProcess.java:282) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.startup.StartupProcess.completeStartupFutureExceptionallySynchronized(StartupProcess.java:183) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.startup.StartupProcess.lambda$proceedWithStartupSynchronized$3(StartupProcess.java:167) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.future.FutureContinuationRunnable.run(FutureContinuationRunnable.java:33) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorJob.invoke(ActorJob.java:94) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorJob.execute(ActorJob.java:45) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorTask.execute(ActorTask.java:119) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorThread.executeCurrentTask(ActorThread.java:106) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorThread.doWork(ActorThread.java:87) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	at io.camunda.zeebe.scheduler.ActorThread.run(ActorThread.java:198) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	Suppressed: io.camunda.zeebe.scheduler.startup.StartupProcessStepException: Bootstrap step Command API failed
		at io.camunda.zeebe.scheduler.startup.StartupProcess.completeStartupFutureExceptionallySynchronized(StartupProcess.java:185) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.startup.StartupProcess.lambda$proceedWithStartupSynchronized$3(StartupProcess.java:167) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.future.FutureContinuationRunnable.run(FutureContinuationRunnable.java:33) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.ActorJob.invoke(ActorJob.java:94) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.ActorJob.execute(ActorJob.java:45) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.ActorTask.execute(ActorTask.java:119) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.ActorThread.executeCurrentTask(ActorThread.java:106) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.ActorThread.doWork(ActorThread.java:87) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.ActorThread.run(ActorThread.java:198) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
	Caused by: java.lang.NullPointerException: Cannot invoke "io.camunda.zeebe.broker.system.monitoring.DiskSpaceUsageMonitor.addDiskUsageListener(io.camunda.zeebe.broker.system.monitoring.DiskSpaceUsageListener)" because the return value of "io.camunda.zeebe.broker.bootstrap.BrokerStartupContext.getDiskSpaceUsageMonitor()" is null
		at io.camunda.zeebe.broker.bootstrap.CommandApiServiceStep.lambda$startCommandApiService$2(CommandApiServiceStep.java:91) ~[zeebe-broker-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.broker.bootstrap.AbstractBrokerStartupStep.forwardExceptions(AbstractBrokerStartupStep.java:71) ~[zeebe-broker-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.broker.bootstrap.AbstractBrokerStartupStep.lambda$proceed$3(AbstractBrokerStartupStep.java:87) ~[zeebe-broker-8.1.0.jar:8.1.0]
		at io.camunda.zeebe.scheduler.future.FutureContinuationRunnable.run(FutureContinuationRunnable.java:28) ~[zeebe-scheduler-8.1.0.jar:8.1.0]
		... 6 more```

</p>
</details>

**Environment:**
- OS: <!-- [e.g. Linux] -->
- Zeebe Version: 8.1.0
- Configuration: `ZEEBE_BROKER_DATA_DISKUSAGEMONITORINGENABLED=false`

[oleschoenburg]

Looks like there could be more components that would fail if disk usage monitoring is enabled so just adding a null check to the command api step is not enough.