plugins: allow empty config

pkg/plugins/middlewares.go

@@ -84,6 +84,9 @@ func (p middlewareBuilder) createConfig(config map[string]interface{}) (reflect.
 	}
 
 	vConfig := results[0]
+	if len(config) == 0 {
+		return vConfig, nil
+	}
 
 	cfg := &mapstructure.DecoderConfig{
 		DecodeHook:       mapstructure.StringToSliceHookFunc(","),