Fix exception in alert for inavlid parameter

lib/renderer/override.js

@@ -154,20 +154,11 @@ window.open = function (url, frameName, features) {
 }
 
 // Use the dialog API to implement alert().
-window.alert = function (message, title) {
+window.alert = function (message = '', title = '') {
-  var buttons
-  if (arguments.length === 0) {
-    message = ''
-  }
-  if (title == null) {
-    title = ''
-  }
-  buttons = ['OK']
-  message = String(message)
   remote.dialog.showMessageBox(remote.getCurrentWindow(), {
-    message: message,
+    message: String(message),
-    title: title,
+    title: String(title),
-    buttons: buttons
+    buttons: ['OK']
   })
 }