add empty_featureset (returning feature_ptr()) implementation

include/mapnik/featureset.hpp

@@ -41,8 +41,23 @@ struct MAPNIK_DECL Featureset : private util::noncopyable
     virtual ~Featureset() {}
 };
 
+
+struct MAPNIK_DECL empty_featureset final : Featureset
+{
+    feature_ptr next()
+    {
+        return feature_ptr();
+    }
+    ~empty_featureset() {}
+};
+
 using featureset_ptr = std::shared_ptr<Featureset>;
 
+inline featureset_ptr make_empty_featureset()
+{
+    return std::make_shared<empty_featureset>();
+}
+
 }
 
 #endif // MAPNIK_FEATURESET_HPP