fix: set default broadcaster to null

Author: ricardogobbosouza

README.md

@@ -66,7 +66,7 @@ export default {
 ### `broadcaster`
 
 - Type: `String`
-- Default: `undefined`
+- Default: `'null'`
 
 You can use `'pusher'`, `'socket.io'` or `'null'`.
 

lib/module.js

@@ -3,6 +3,7 @@ const { resolve } = require('path')
 module.exports = function (moduleOptions) {
   this.nuxt.hook('builder:extendPlugins', (plugins) => {
     const options = {
+      broadcaster: 'null',
       authModule: false,
       connectOnLogin: true,
       disconnectOnLogout: true,