"arRenderer" can be undefined in Controls.ts

Since the latest release `v4.2.0`, this new error may occur:

```
Cannot read properties of null (reading 'isPresenting')
```
Related to an undefined value in `controls.ts` > `[$syncCameraTarget]` > `arRenderer`:
https://github.com/google/model-viewer/blob/edf2f3f279939041fdeb7dbef9eb524255ed96fc/packages/model-viewer/src/features/controls.ts#L756

Reproduce on the following rowsers (issue tracked by Sentry):
- Chrome 146.0.0
- Chrome 145.0.0
- Chrome 144.0.0
- Chrome 142.0.0
- Firefox 149.0
- Firefox 148.0
- Firefox 147.0
- Edge 146.0.0
- ...

### Stack Trace
```
TypeError: Cannot read properties of null (reading 'isPresenting')
at y.(../../node_modules/@google/model-viewer/lib/features/controls.js:513:45)
at .value(../../node_modules/@google/model-viewer/lib/decorators.js:86:33)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:96:30)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:98:37)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:98:37)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:98:37)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:98:37)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:98:37)
at i.updated(../../node_modules/@google/model-viewer/lib/features/environment.js:51:19)
at i.updated(../../node_modules/@google/model-viewer/lib/features/staging.js:68:19)
at .updated.value(../../node_modules/@google/model-viewer/lib/decorators.js:98:37)
at i.updated(../../node_modules/@google/model-viewer/lib/features/scene-graph.js:105:19)
at y._$AE(../../node_modules/@lit/reactive-element/reactive-element.js:6:5456)
at y.performUpdate(../../node_modules/@lit/reactive-element/reactive-element.js:6:5320)
at y.scheduleUpdate(../../node_modules/@lit/reactive-element/reactive-element.js:6:4775)
at y._$EP(../../node_modules/@lit/reactive-element/reactive-element.js:6:4683)
at y._$EP(../../node_modules/@lit/reactive-element/reactive-element.js:6:4627)
```