Model::find(NULL) throws an exception

The find method doesn't support NULL as parameter. The expected behavior is to return NULL (as Eloquent does).

```
"error": {
  "type":"ErrorException",
  "message":"Undefined index: value",
  "file":"...\/vendor\/jenssegers\/mongodb\/src\/Jenssegers\/Mongodb\/Builder.php",
  "line":587
}
```

I don't see a simple patch right now. I just start using your package and I hope help you more later.
