hasOne only returns null if *all* records don't have a relation

Issue Description

Following this PR change, we're expecting that all empty hasOne relations to be null, but it seems that that only applies if all instances of the result have an empty relationship. If even any result does have related data then the records with no relationship get set to empty object.

Steps to reproduce issue

Modify the existing test 'does not load "hasOne" relationship when it doesn\'t exist (site -> meta)' to fetch all Sites and run assertion against each record returned.

it('does not load "hasOne" relationship when it doesn\'t exist (site -> meta)', function() {
    return Site.fetchAll({
        withRelated: ['meta']
    }).then(function(site) {
        const sites = site.toJSON();
        for (const site of sites) {
            expect(site).to.not.have.property('meta');
        }
    });
});

Expected behaviour

The third record in the response should be { id: 3, name: 'backbonejs.org' } (no meta property)

Actual behaviour

[ { id: 1,
    name: 'knexjs.org',
    meta:
     { id: 1,
       site_id: 1,
       description: 'This is a description for the Knexjs Site' } },
  { id: 2,
    name: 'bookshelfjs.org',
    meta:
     { id: 2,
       site_id: 2,
       description: 'This is a description for the Bookshelfjs Site' } },
  { id: 3, name: 'backbonejs.org', meta: {} } ]

Just to confirm, I updated the fixtures to insert one more Site with the name 'backbonejs.org', then updated the fetch to get all records with that name, and this is the serialized response:

[ { id: 3, name: 'backbonejs.org' },
  { id: 4, name: 'backbonejs.org' } ]

Is this desired/expected behaviour so that all objects match? I understand that, but it is a bit confusing that fetches could return differently shaped objects. This might just be a documentation peice rather than an actual issue.

bookshelf / bookshelf