Fix belongsTo relationships to models with id 0

I fixed the following failing test:
```
Module Failed: DS.belongsTo
  Test Failed: belongsTo supports relationships to models with id 0
    Assertion Failed: the tag property should return a tag
      Expected: true, Actual: false   
    Assertion Failed: the tag shuld have name
      Expected: friendly, Actual: null   
    Assertion Failed: relationship object is the same as object retrieved directly
      Expected: <Tag:ember8891:0>, Actual: null
```