enable nodeExternals for modulesDir if exists

#1076

lib/builder/webpack/server.config.js

@@ -3,6 +3,7 @@ import VueSSRServerPlugin from 'vue-server-renderer/server-plugin'
 import nodeExternals from 'webpack-node-externals'
 import { each } from 'lodash'
 import { resolve } from 'path'
+import { existsSync } from 'fs'
 import base from './base.config.js'
 
 /*
@@ -34,15 +35,7 @@ export default function webpackServerConfig () {
       hints: false,
       maxAssetSize: Infinity
     },
-    externals: [
+    externals: [],
-      // https://webpack.js.org/configuration/externals/#externals
-      // https://github.com/liady/webpack-node-externals
-      nodeExternals({
-        // load non-javascript files with extensions, presumably via loaders
-        whitelist: [/\.(?!(?:js|json)$).{1,5}$/i],
-        modulesDir: this.options.modulesDir
-      })
-    ],
     plugins: (config.plugins || []).concat([
       new VueSSRServerPlugin({
         filename: 'server-bundle.json'
@@ -56,6 +49,16 @@ export default function webpackServerConfig () {
     ])
   })
 
+  // https://webpack.js.org/configuration/externals/#externals
+  // https://github.com/liady/webpack-node-externals
+  if (existsSync(this.options.modulesDir)) {
+    config.externals.push(nodeExternals({
+      // load non-javascript files with extensions, presumably via loaders
+      whitelist: [/\.(?!(?:js|json)$).{1,5}$/i],
+      modulesDir: this.options.modulesDir
+    }))
+  }
+
   // --------------------------------------
   // Production specific config
   // --------------------------------------