Fixes exeption if `map` return null or undefined

```javascript
var Foo = Backbone.AssociatedModel.extend({});

        var Bar = Backbone.AssociatedModel.extend({
            relations: [
                {
                    type: Backbone.One,
                    key: 'rel',
                    relatedModel: Foo,
                    map: function(m) {
                        if(m.rel.test == '') return null;
                        return m;
                    }
                }
            ],
        });

        var bar = new Bar({rel: {'test': ''}});
```
this code raised exeption:
```
Uncaught TypeError: Cannot call method 'hasOwnProperty' of null backbone-associations.js:314
(anonymous function) backbone-associations.js:314
_.each._.forEach underscore.js:79
Backbone.AssociatedModel.Backbone.Associations.AssociatedModel.BackboneModel.extend._setAttr backbone-associations.js:259
Backbone.AssociatedModel.Backbone.Associations.AssociatedModel.BackboneModel.extend._set backbone-associations.js:233
Backbone.AssociatedModel.Backbone.Associations.AssociatedModel.BackboneModel.extend.set backbone-associations.js:197
Backbone.Model backbone-min.js:266
Backbone.AssociatedModel.Backbone.Associations.AssociatedModel.BackboneModel.extend.constructor backbone-associations.js:115
Backbone.AssociatedModel.extend._.extend.constructor notified-model.js:7
child backbone-min.js:1583
_.extend._prepareModel backbone-min.js:929
_.extend.set backbone-min.js:720
_.each.CollectionProto.(anonymous function) backbone-associations.js:795
_.extend.add backbone-min.js:652
_.extend.reset backbone-min.js:783
_.each.CollectionProto.(anonymous function) backbone-associations.js:795
_.extend.fetch.options.success backbone-min.js:879
x.Callbacks.c jquery-1.10.1.min.js:745
x.Callbacks.p.fireWith jquery-1.10.1.min.js:779
k jquery-1.10.1.min.js:1872
x.ajaxTransport.send.r
```