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(","),

pkg/server/middleware/plugins.go

@@ -2,7 +2,6 @@ package middleware
 
 import (
 	"errors"
-	"fmt"
 
 	"github.com/traefik/traefik/v2/pkg/config/dynamic"
 	"github.com/traefik/traefik/v2/pkg/plugins"
@@ -30,9 +29,5 @@ func findPluginConfig(rawConfig map[string]dynamic.PluginConf) (string, map[stri
 		return "", nil, errors.New("missing plugin type")
 	}
 
-	if len(rawPluginConfig) == 0 {
-		return "", nil, fmt.Errorf("missing plugin configuration: %s", pluginType)
-	}
-
 	return pluginType, rawPluginConfig, nil
 }