computed.sort is not updated if the value of the itemsKey is null

[pieter-v]

If the first time you 'get' the sorted property while the property itself is null, then the sorted property is never recalculated if the property receives a value afterwards (see attached file for a reproduction scenario).

works on 2.15.3, but fails on 2.16.0

The reason why this fails in our app, is that property is a sub-property of model (model.users and model.sortedUsers) and that rendering starts between the beforeModel and model hook. So, the value controller.model is still null.
sorting.zip

[rwjblue]

I believe that this was fixed by #15733.

[pieter-v]

Thanks