Exit before pushing a trace event when layer tree holder is empty

Author: Kaushik Iska

shell/common/rasterizer.cc

@@ -120,6 +120,10 @@ void Rasterizer::DrawLastLayerTree() {
 }
 
 void Rasterizer::Draw(std::shared_ptr<LayerTreeHolder> layer_tree_holder) {
+  // We do not yet have any frame to raster.
+  if (layer_tree_holder->IsEmpty()) {
+    return;
+  }
   TRACE_EVENT0("flutter", "GPURasterizer::Draw");
   if (raster_thread_merger_ &&
       !raster_thread_merger_->IsOnRasterizingThread()) {