This reduces the complexity involved in making the row count a
property. Empty models do have rows like this, but don't have a height.
It doesn't get much more consistent than that. Before empty models
didn't have rows but you could still query the row heights. Having
height == 0 is very helpful for assembling the UI, rows == 0 not so
much.

Change-Id: I38ee9f46751a4beb288578d5cd1f0a17ea08814a
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>