[shell] Ignore placeholder screens

A placeholder screen is created by Qt when no real screens exist. We
don't want to create panels and containments for those, it is a whole
world of pain.

Plasma has a hook to ignore some screens which is extended with this
check.

CCBUG: 419492

shell/shellcorona.cpp

@@ -1126,6 +1126,9 @@ bool ShellCorona::isOutputRedundant(QScreen *screen) const
         if (screen == s) {
             continue;
         }
+        if (s->geometry().isNull()) {
+            return false;
+        }
 
         const QRect otherGeometry = s->geometry();