A hack to greatly improve load times

I was investigating
gazebosim/gazebo_test_cases#1576 , in my
investigation it came to my notice that `sdf::Element` takes forever to
destroy (We should open a ticket somewhere about this). If we are
skipping serialization we might as well not create and destroy an
SDF Element. This hack greatly speeds up the load time for gazebo.

Signed-off-by: Arjo Chakravarty <[email protected]>

include/gz/sim/components/Model.hh

@@ -75,6 +75,7 @@ namespace serializers
         }
       }
 
+      // Why bother even sending this. Wouldn't a blank string work?
       _out << "<?xml version=\"1.0\" ?>"
            << "<sdf version='" << SDF_PROTOCOL_VERSION << "'>"
            << (skip ? std::string() : modelElem->ToString(""))
@@ -91,11 +92,17 @@ namespace serializers
     {
       sdf::Root root;
       std::string sdf(std::istreambuf_iterator<char>(_in), {});
+      if (sdf.find("model") == std::string::npos)
+      {
+        gzwarn << "No model was sent\n";
+        return _in;
+      }
+      // Its super expensive to create an SDFElement for some reason
 
       sdf::Errors errors = root.LoadSdfString(sdf);
       if (!root.Model())
       {
-        gzwarn << "Unable to deserialize sdf::Model" << std::endl;
+        gzwarn << "Unable to deserialize sdf::Model " << sdf<< std::endl;
         return _in;
       }