fix: null check

[ikalachy]

The following exception potentially breaks the hazelcast cluster and we need to restart the cluster + approuter.

java.lang.NullPointerException: Cannot invoke "io.vertx.core.json.JsonArray.copy()" because the return value of "java.util.Map.remove(Object)" is null,
at io.neonbee.internal.cluster.entity.ClusterEntityRegistry.lambda$unregisterNode$1(ClusterEntityRegistry.java:127),

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud