[greeter] Dont quit when last window closes

Qt allows to have no screens. If all XRandR screens are disconnected
there are also no QScreens. This is from an X11 perspective wrong, but
we have to deal with it. No QScreens means that all views are destroyed
and no new ones are created. Thus all windows close and the greeter
exited successfully which in turn unlocked the screen.

This change ensures that the greeter doesn't exit when all windows close.

Funnily this was not a problem with the well known issue of Qt
crashing when all screens are disconnected. The lock screen handles
crashes gracefully and just restarts or more likely Qt also crashes
the application using KSld and thus taking down the complete session.

CCBUG: 358125