qmakeproject: fix crash when qtVersion is null

While it is not possible to create a qmake kit without qt, the qt
version might become invalid and be removed.

Change-Id: I99484c86d731ab5a47bff9bb1958158504617004
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>