Keyboard navigation is broken

Keyboard navigation (using `ArrowLeft` and `ArrowRight` to go to the previous or next topic) is broken on any site created using this addon. You can check it out by going to any topic on this addons own docs and pressing one of the arrow keys.

The resulting error is something like this:
```
Uncaught Error: More context objects were passed than there are dynamic segments for the route: docs.usage
at I.applyToHandlers (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:46691:15)
at I.applyToState (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:46628:19)
at s.applyIntent (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:47510:21)
at R (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:28701:45)
at r._prepareQueryParams (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:28260:19)
at r._doTransition (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:28219:12)
at h.transitionTo (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:27027:37)
at i.nextPage (vendor-907f1d49045ce1b9b3f2f6d948b40f39.js:81107:23)
at chunk.21.0381709c1e24f2a2857d.js:2:117294
at Array.forEach ()
```

The offending code seems to be at https://github.com/ember-learn/ember-cli-addon-docs/blob/daeaf9f909328ba92b106031b992bb085b96c23d/addon/components/docs-viewer/index.js#L55..L56 and https://github.com/ember-learn/ember-cli-addon-docs/blob/daeaf9f909328ba92b106031b992bb085b96c23d/addon/components/docs-viewer/index.js#L66..L67.

The fix seems fairly easy; If the `model` on a route is `undefined` the `transitionTo` method should not be called with the model argument.

Changing the code to the following fixes it, but I don't know if this might have side effects
```js
const { route, model } = this.docsRoutes.previous;
if (model) {
this.router.transitionTo(route, model);
} else {
this.router.transitionTo(route);
}
```

Inspect addon/components/docs-viewer/index.js at the referenced lines and reproduce the failure by using ArrowLeft or ArrowRight on a topic in the addon documentation. Check the previous and next route transitions when the model is undefined. Done means both keyboard directions navigate successfully without the dynamic-segments error.

Scritto dal modello di indicizzazione a partire dal testo della issue.