InfinityLoader.willDestroy incorrectly assumes `inifinityModel` is still valid

After updating to ember-infinity 3.0.0 I'm seeing the following error when transitioning to a new route:

```
global failure: TypeError: Cannot read properties of undefined (reading 'off')
Source:
TypeError: Cannot read properties of undefined (reading 'off')
```

The error occurs in `addon/components/infinity-loader.js#willDestroy` because `infinityModel` is `null`. I see that in commit
https://github.com/adopted-ember-addons/ember-infinity/commit/9b6956b6714fd5fd156183652fa7efd646eddbd4 the check for `isDestroyed` was added because of the same error after a route change.

The difference in my situation is that `isDestroyed` is `false` BUT `isDestroying` is `true`. In my case the `infinityModel` has already been nullified at this point.

### Context

The application is running Ember 3.28 and is being prepared for an upgraded to 4. I tested the same code with Ember 4.0.1 but got the same result.