Fix NPE caused by MultiStageQueryThrottler

[jackjlli]

This PR fixes the NPE which is caused by MultiStageQueryThrottler being null when multi-stage engine feature is disabled.

To be more specific, when the multi-stage engine is disabled, the MultiStageQueryThrottler reference will be null. This makes the ClusterChangeMediator fail to process cluster changes like cluster config changes and external view changes.

pinot/pinot-broker/src/main/java/org/apache/pinot/broker/broker/helix/BaseBrokerStarter.java

Line 344 in 25fc746

if (_brokerConf.getProperty(Helix.CONFIG_OF_MULTI_STAGE_ENGINE_ENABLED, Helix.DEFAULT_MULTI_STAGE_ENGINE_ENABLED)) {

Sample error messages:

2025/01/09 19:43:44.610 ERROR [ClusterChangeMediator] [ClusterChangeHandlingThread] [pinot-broker] [] Caught exception within cluster change handling thread
java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "changeHandler" is null
        at org.apache.pinot.broker.broker.helix.ClusterChangeMediator.processClusterChange(ClusterChangeMediator.java:134) ~[org.apache.pinot.pinot-broker-1.3.0-dev-1461.jar:...]
        at org.apache.pinot.broker.broker.helix.ClusterChangeMediator.lambda$new$0(ClusterChangeMediator.java:98) ~[org.apache.pinot.pinot-broker-1.3.0-dev-1461.jar:...]
        at java.lang.Thread.run(Thread.java:833) [?:?]