Improve load times by skipping serialization of entities when unecessary. (#2596) (#2683)

* 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 <arjoc@intrinsic.ai>
Signed-off-by: Ian Chen <ichen@openrobotics.org>
Co-authored-by: Ian Chen <ichen@openrobotics.org>
(cherry picked from commit 1a88131)

Author: mergify[bot]

include/gz/sim/components/Model.hh

@@ -75,10 +75,18 @@ namespace serializers
         }
       }
 
-      _out << "<?xml version=\"1.0\" ?>"
-           << "<sdf version='" << SDF_PROTOCOL_VERSION << "'>"
-           << (skip ? std::string() : modelElem->ToString(""))
-           << "</sdf>";
+      if (!skip)
+      {
+        _out << "<?xml version=\"1.0\" ?>"
+              << "<sdf version='" << SDF_PROTOCOL_VERSION << "'>"
+              << modelElem->ToString("")
+              << "</sdf>";
+
+      }
+      else
+      {
+        _out << "";
+      }
       return _out;
     }
 
@@ -89,13 +97,18 @@ namespace serializers
     public: static std::istream &Deserialize(std::istream &_in,
                 sdf::Model &_model)
     {
-      sdf::Root root;
       std::string sdf(std::istreambuf_iterator<char>(_in), {});
+      if (sdf.empty())
+      {
+        return _in;
+      }
 
+      // Its super expensive to create an SDFElement for some reason
+      sdf::Root root;
       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;
       }