Using browseable API to update Interface object with empty attributes raises 400

The attributes field should default to an empty object (`{}`) but it's blank. So if the attributes for an Interface object are unset, upon clicking "PUT" to update the instance, a 400 is raised from the browseable API:

```javascript

PUT /api/sites/1/interfaces/foo14-ab01:s45/

HTTP 400 Bad Request
Allow: GET, PUT, PATCH, DELETE, OPTIONS
Content-Type: application/json
Vary: Accept

{
"error": {
"message": {
"attributes": "Expected dictionary but received "
},
"code": 400
}
}
```

Strangely despite the error, any update still succeeds (such as if you're updating the `description` field). Also, if one puts `{}` into the `attributes` field and submits with the "PUT" button, the operation succeeds without any error.

There's something fishy going on here and appears to be two issues:
- The default value for the `attributes` field should be `{}` if the field is empty.
- The validation of the attributes field appears to block only update to that field and not update to the entire object.