[fix][broker] Fix NPE after publishing a tombstone to the service uni…

…t channel

### Motivation

NPE will happen in `UnloadManager#handleEvent` after
apache#22743. It's because the `Init`
state is always associated with a null `ServiceUnitStateData`.

```
java.lang.NullPointerException: Cannot invoke "org.apache.pulsar.broker.loadbalance.extensions.channel.ServiceUnitStateData.force()" because "data" is null
        at org.apache.pulsar.broker.loadbalance.extensions.manager.UnloadManager.handleEvent(UnloadManager.java:204) ~[classes/:?]
        at org.apache.pulsar.broker.loadbalance.extensions.channel.StateChangeListeners.lambda$notify$3(StateChangeListeners.java:74) ~[classes/:?]
        at java.base/java.util.concurrent.CopyOnWriteArrayList.forEach(CopyOnWriteArrayList.java:807) ~[?:?]
        at org.apache.pulsar.broker.loadbalance.extensions.channel.StateChangeListeners.notify(StateChangeListeners.java:72) ~[classes/:?]
        at org.apache.pulsar.broker.loadbalance.extensions.channel.ServiceUnitStateChannelImpl.handleInitEvent(ServiceUnitStateChannelImpl.java:902) ~[classes/:?]
        at org.apache.pulsar.broker.loadbalance.extensions.channel.ServiceUnitStateChannelImpl.handle(ServiceUnitStateChannelImpl.java:715) ~[classes/:?]
```

### Modifications

In `UnloadManager#handleEvent`, assume `data` is null and call
`complete` directly. Fix `UnloadManagerTest`, which passes a non-null
`ServiceUnitStateData` and `Init` to `handleEvent`.

pulsar-broker/src/main/java/org/apache/pulsar/broker/loadbalance/extensions/manager/UnloadManager.java

@@ -22,6 +22,7 @@
 import static org.apache.pulsar.broker.loadbalance.extensions.channel.ServiceUnitState.Owned;
 import static org.apache.pulsar.broker.loadbalance.extensions.models.UnloadDecision.Label.Failure;
 import static org.apache.pulsar.broker.loadbalance.extensions.models.UnloadDecision.Reason.Unknown;
+import static com.google.common.base.Preconditions.checkArgument;
 import com.google.common.annotations.VisibleForTesting;
 import io.prometheus.client.Histogram;
 import java.util.Map;
@@ -201,9 +202,8 @@ public void handleEvent(String serviceUnit, ServiceUnitStateData data, Throwable
                 }
             }
             case Init -> {
-                if (data.force()) {
-                    complete(serviceUnit, t);
-                }
+                checkArgument(data == null, "Init state must be associated with null data");
+                complete(serviceUnit, t);
             }
             case Owned -> complete(serviceUnit, t);
             case Releasing -> LatencyMetric.RELEASE.endMeasurement(serviceUnit);

pulsar-broker/src/test/java/org/apache/pulsar/broker/loadbalance/extensions/manager/UnloadManagerTest.java

@@ -126,11 +126,7 @@ public void testSuccess() throws IllegalAccessException, ExecutionException, Int
         assertEquals(inFlightUnloadRequestMap.size(), 1);
 
         // Success with Init state.
-        manager.handleEvent(bundle,
-                new ServiceUnitStateData(ServiceUnitState.Init, null, srcBroker, false, VERSION_ID_INIT), null);
-        assertEquals(inFlightUnloadRequestMap.size(), 1);
-        manager.handleEvent(bundle,
-                new ServiceUnitStateData(ServiceUnitState.Init, null, srcBroker, true, VERSION_ID_INIT), null);
+        manager.handleEvent(bundle, null, null);
         assertEquals(inFlightUnloadRequestMap.size(), 0);
         future.get();
         assertEquals(counter.getBreakdownCounters().get(Success).get(Admin).get(), 1);