Initialize default shadow caster name in Scene

The shadow caster name in the Scene is empty if the ignition
transport service call fails, so initialize it to the default
value.

Follow up to gazebosim#3048.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

gazebo/rendering/ScenePrivate.hh

@@ -378,7 +378,7 @@ namespace gazebo
       public: std::map<int32_t, bool> layerState;
 
       /// \brief Shadow caster material name
-      public: std::string shadowCasterMaterialName;
+      public: std::string shadowCasterMaterialName = "Gazebo/shadow_caster";
     };
   }
 }