Ember.Select and RecordArrays

Ember.Select checks for the presence of content before registering a change https://github.com/emberjs/ember.js/blob/master/packages/ember-handlebars/lib/controls/select.js#L416.

When used with RecordArrays (which are truthy even with length 0), this will mistakenly set the selection to `null` (https://github.com/emberjs/ember.js/blob/master/packages/ember-handlebars/lib/controls/select.js#L420) changing a  `belongsTo` relationship bound as the selection.

I think checking for `content.length` resolves this. Will PR unless this sounds mega crazy.