QConcatenatedTablesProxyModel should return 0 rows when it has 0 columns

As I was testing here:
https://codereview.qt-project.org/c/qt/qtdeclarative/+/372646/5

If we report a number of rows but 0 columns the views get compelled to request these rows. Obviously we could argue that the view should be checking both but it also makes little sense that we claim to have N rows but nothing to show for.

Furthermore, this behaviour should at least warn the user that something's off, which now it doesn't and you're just left to wonder why things aren't really working.