emptyView shouldn't be constrained by a container height.

After adding the emptyView, I missed the fact that the container still had a height of 0. When the emptyView is displaying the ember-list-container should have no inline height. Updated tests to account for this.

I added `_isChildEmptyView` to determine if the emptyView is actually the current child, which I am now using to determine if we need to remove the emptyView as well. This is an improvement over the check that was there before, since we will only run the more expensive `indexOf` when `emptyView` is an instance of `Ember.View` and the sole childView.