fix: hotfix when module options is not set in config

Author: kevinmarrec

lib/module.js

@@ -13,7 +13,7 @@ module.exports = function (moduleOptions) {
   this.nuxt.hook('build:before', () => {
     const options = merge.all([
       defaults,
-      this.options.vuetify,
+      this.options.vuetify || {},
       moduleOptions
     ])