If SurfaceItem does not have a pixmap, return an empty WindowQuadList

Otherwise the caller needs to ensure that we're not calling on an item
without pixmap and if we don't, things crash.

src/surfaceitem.cpp

@@ -127,6 +127,10 @@ void SurfaceItem::preprocess()
 
 WindowQuadList SurfaceItem::buildQuads() const
 {
+    if (!pixmap()) {
+        return {};
+    }
+
     const QRegion region = shape();
     const auto size = pixmap()->size();