NPE when processing some AV events

```
java.lang.NullPointerException: Parameter specified as non-null is null: method net.nemerosa.ontrack.extension.notifications.subscriptions.EntitySubscriptionStore.forEachByEvent, parameter projectEntity
at net.nemerosa.ontrack.extension.notifications.subscriptions.EntitySubscriptionStore.forEachByEvent(EntitySubscriptionStore.kt)
```