fix: service worker path

Author: yyx990803

lib/webpack/createBaseConfig.js

@@ -221,7 +221,7 @@ module.exports = function createBaseConfig ({
   config
     .plugin('injections')
     .use(require('webpack/lib/DefinePlugin'), [{
-      BASE_URL: JSON.stringify(siteConfig.base),
+      BASE_URL: JSON.stringify(siteConfig.base || '/'),
       GA_ID: siteConfig.ga ? JSON.stringify(siteConfig.ga) : false,
       SW_ENABLED: !!siteConfig.serviceWorker
     }])