Initialize default shadow caster name in Scene (#3067)

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 #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";
     };
   }
 }