bugfix: Cannot set property 'id' of null

Sometimes `this.shareDoc.snapshot === null` (if there is no such a doc but we fetch/subscribe on it: `model.at('items.id1').fetch())`
But! 
```js
typeof null === "object" // true!!!!!!
```
So, it's truly condition:
```js
  if (typeof snapshot === 'object' && !Array.isArray(snapshot)) {
    snapshot.id = this.id;
  }
  // here we got:
  // null.id = this.id
```