xwayland: Do not set scale to 0

It would be the case upon lid close which would make KWin crash. Also
adds an assert so in case it happened again we would be able to catch it
easily. It gets applied asynchronously so it needs to be chased down.

src/main.cpp

@@ -381,6 +381,7 @@ static X11EventFilterContainer *takeEventFilter(X11EventFilter *eventFilter,
 
 void Application::setXwaylandScale(qreal scale)
 {
+    Q_ASSERT(scale != 0);
     if (scale != m_xwaylandScale) {
         m_xwaylandScale = scale;
         // change the font dpi to match

src/wayland/clientconnection.cpp

@@ -155,6 +155,7 @@ QString ClientConnection::executablePath() const
 
 void ClientConnection::setScaleOverride(qreal scaleOveride)
 {
+    Q_ASSERT(scaleOveride != 0);
     d->scaleOverride = scaleOveride;
     Q_EMIT scaleOverrideChanged();
 }

src/workspace.cpp

@@ -477,7 +477,7 @@ bool Workspace::applyOutputConfiguration(const OutputConfiguration &config, cons
     KConfig cfg(QStringLiteral("kdeglobals"));
     KConfigGroup kscreenGroup = cfg.group("KScreen");
     const bool xwaylandClientsScale = kscreenGroup.readEntry("XwaylandClientsScale", true);
-    if (xwaylandClientsScale) {
+    if (xwaylandClientsScale && !outputOrder.isEmpty()) {
         double maxScale = 0;
         for (Output *output : outputOrder) {
             const auto changeset = config.constChangeSet(output);