[push-api] .getRegistration() should not reject if there is no registration (but return a null object) (#96)

CC @jakearchibald

It seems to be the pattern generally being used with Promises. Rejecting should be an exceptional case. In that case, having no registration isn't exceptional, it's definitely expected.

Also, as @jakearchibald pointed, it would allow to do things like that in ES7:
```javascript
var reg = await push.getRegistration();
```

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https://github.com/w3c/push-api/issues/96