…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`.