[ksplash][logout-greeter] Avoid making windows for placeholder screens

A placeholder screen is created by Qt when no wl_outputs exist even for
a split second which can happen when plugging and unplugging monitors
due to driver order.

ksplash/ksplashqml/splashapp.cpp

@@ -118,6 +118,9 @@ void SplashApp::setStage(int stage)
 
 void SplashApp::adoptScreen(QScreen *screen)
 {
+    if (screen->geometry().isNull()) {
+        return;
+    }
     SplashWindow *w = new SplashWindow(m_testing, m_window, m_theme);
     w->setGeometry(screen->geometry());
     w->setScreen(screen);

logout-greeter/greeter.cpp

@@ -56,6 +56,9 @@ void Greeter::init()
 
 void Greeter::adoptScreen(QScreen *screen)
 {
+    if (screen->geometry().isNull()) {
+        return;
+    }
     // TODO: last argument is the theme, maybe add command line option for it?
     KSMShutdownDlg *w = new KSMShutdownDlg(nullptr, m_shutdownType);
     w->installEventFilter(this);