fix: ensure runnable when no locales are provided

Author: yyx990803

lib/app/dataMixin.js

@@ -34,7 +34,7 @@ export default {
       return store.siteData
     },
     $localeConfig () {
-      const { locales } = this.$site
+      const { locales = {}} = this.$site
       let targetLang
       let defaultLang
       Object.keys(locales).forEach(path => {