Reduce/Eliminate sdf::Model and sdf::World serialization warnings

- Reduces serialization warnings on `sdf::Model` so it only occurs when
  the `relative_to` attribute is set. This was the intended behavior
  originally, but there was a bug in the code.
- Eliminate `sdf::World` warnings by specializing the `DefaultSerializer` for `sdf::World`.

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

include/gz/sim/components/Model.hh

@@ -54,7 +54,7 @@ namespace serializers
       if (modelElem->HasElement("pose"))
       {
         sdf::ElementPtr poseElem = modelElem->GetElement("pose");
-        if (poseElem->HasAttribute("relative_to"))
+        if (poseElem->GetAttribute("relative_to")->GetSet())
         {
           // Skip serializing models with //pose/@relative_to attribute
           // since deserialization will fail. This could be a nested model.

include/gz/sim/components/World.hh

@@ -29,6 +29,29 @@ namespace sim
 {
 // Inline bracket to help doxygen filtering.
 inline namespace GZ_SIM_VERSION_NAMESPACE {
+
+namespace serializers
+{
+/// \brief Specialize the DefaultSerializer on sdf::World so we can
+/// skip serialization
+/// TODO(azeey) Do we ever want to serialize this component?
+template <>
+class DefaultSerializer<sdf::World>
+{
+  public:
+  static std::ostream &Serialize(std::ostream &_out, const sdf::World &)
+  {
+    return _out;
+  }
+
+  public:
+  static std::istream &Deserialize(std::istream &_in, sdf::World &)
+  {
+    return _in;
+  }
+};
+}
+
 namespace components
 {
   /// \brief A component that identifies an entity as being a world.