Creating a record without content fails, but reports success #387

Steps to reproduce

Create a record with no content:

    var task = this.store.createRecord('task', {});
    console.log(task.get('currentState.stateName'));  // root.loaded.created.uncommitted
    task.save().then(data => {
      console.log(data.get('currentState.stateName'));  // root.loaded.saved
    });
    console.log(task.get('currentState.stateName'));  // root.loaded.created.inFlight

An ID is assigned in the ED store, and the record reports having been saved successfully, but no data has actually been recorded in Firebase.

Expected behavior

Any empty record should be returned when fetching the list of Tasks

Actual behavior

The new Task is not saved, unless data is added.

Pretty sure this occurs because there is no way to store an empty record in Firebase -- it just deletes the node. In the ED world, though, that's a surprising result -- usually fields are optional, and there's a big difference between a record being created with no attributes set vs no record at all

I see two workable options:

1. use some sort of sentinel value as a default (false?), that gets interpreted as null when loaded by the adapter
2. fail loudly with an explicit error stating empty objects aren't supported in EmberFire