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

* 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>

* Remove magic word

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

* Send empty string instead of using sentinel value.

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

* Style

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

* fix custom sensor system example build (#2649)

Signed-off-by: Ian Chen <ichen@openrobotics.org>

* remove stray change

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

---------

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)

# Conflicts:
#	include/gz/sim/components/Model.hh

Author: arjo129

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 +97,22 @@ 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())
       {
+<<<<<<< HEAD
         ignwarn << "Unable to deserialize sdf::Model" << std::endl;
+=======
+        gzwarn << "Unable to deserialize sdf::Model " << sdf<< std::endl;
+>>>>>>> 1a881310c (Improve load times by skipping serialization of entities when unecessary. (#2596))
         return _in;
       }