Compositor: dont show exection time if node is unconnected

This is in line with what Geometry Nodes do

NOTE: this originated as a "workaround" for #147752
( but does not tackle its root cause at all... that one is
already fixed with 577a66aea2 )

Pull Request: https://projects.blender.org/blender/blender/pulls/147802

source/blender/editors/space_node/node_draw.cc

@@ -2376,6 +2376,9 @@ static std::optional<std::chrono::nanoseconds> compositor_node_get_execution_tim
   if (const timeit::Nanoseconds *execution_time =
           tree_draw_ctx.compositor_per_node_execution_time->lookup_ptr(key))
   {
+    if (execution_time->count() == 0) {
+      return std::nullopt;
+    }
     return *execution_time;
   }