merge: #8014

8014: fix NPE in startup r=deepthidevaki a=pihme

## Description

Fixes NPE during startup in case a parallel shutdown was running

## Related issues

closes #7991

## Review Hints
Fixed directly in `stable/1.2` because this error cannot should not happen in develop. On develop, startup is interrupted between startup steps, and only then does shutdown start



Co-authored-by: pihme <pihme@users.noreply.github.com>

broker/src/main/java/io/camunda/zeebe/broker/Broker.java

@@ -140,7 +140,9 @@ private void internalStart() {
     try {
       closeProcess = startProcess.start();
       startFuture.complete(this);
-      healthCheckService.setBrokerStarted();
+      if (healthCheckService != null) { // can be null if shutdown was called concurrently
+        healthCheckService.setBrokerStarted();
+      }
     } catch (final Exception bootStrapException) {
       final BrokerCfg brokerCfg = getConfig();
       LOG.error(