Fix connection event error

[coderzc]

Motivation

2025-04-29T14:35:42,091+0000 [broker-client-shared-internal-executor-5-1] ERROR io.streamnative.pulsar.handlers.mqtt.common.systemtopic.SystemTopicBasedSystemEventService - refresh cache error
java.lang.NullPointerException: null
	at java.base/java.util.concurrent.ConcurrentHashMap.putVal(Unknown Source) ~[?:?]
	at java.base/java.util.concurrent.ConcurrentHashMap.put(Unknown Source) ~[?:?]
	at io.streamnative.pulsar.handlers.mqtt.common.MQTTConnectionManager$ConnectEventListener.onChange(MQTTConnectionManager.java:129) ~[?:?]
	at io.streamnative.pulsar.handlers.mqtt.common.systemtopic.SystemTopicBasedSystemEventService.lambda$refreshCache$14(SystemTopicBasedSystemEventService.java:240) ~[?:?]
	at java.base/java.util.ArrayList.forEach(Unknown Source) ~[?:?]
	at io.streamnative.pulsar.handlers.mqtt.common.systemtopic.SystemTopicBasedSystemEventService.refreshCache(SystemTopicBasedSystemEventService.java:240) ~[?:?]
	at io.streamnative.pulsar.handlers.mqtt.common.systemtopic.SystemTopicBasedSystemEventService.lambda$readEvent$13(SystemTopicBasedSystemEventService.java:196) ~[?:?]
	at java.base/java.util.concurrent.CompletableFuture.uniWhenComplete(Unknown Source) ~[?:?]
	at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(Unknown Source) ~[?:?]
	at java.base/java.util.concurrent.CompletableFuture.postComplete(Unknown Source) ~[?:?]
	at java.base/java.util.concurrent.CompletableFuture.complete(Unknown Source) ~[?:?]
	at org.apache.pulsar.client.impl.ConsumerBase.lambda$completePendingReceive$0(ConsumerBase.java:330) ~[io.streamnative-pulsar-client-original-3.3.5.5.jar:3.3.5.5]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) ~[?:?]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) ~[?:?]
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[io.netty-netty-common-4.1.119.Final.jar:4.1.119.Final]
	at java.base/java.lang.Thread.run(Unknown Source) [?:?]

Modifications

Using REMOTE_CONNECTION_SOLT instead of nll value， since ConcurrentMap can't allow null value

Verifying this change

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

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[github-actions]

@coderzc:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)