Issue 5039 - add support of nullable non-object values in settings

lib/prefs.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 28 matching lines @@
   update_last_error: 0,
   update_soft_expiration: 0,
   update_hard_expiration: 0,
   currentVersion: "0.0",
   notificationdata: {},
   notificationurl: "https://notification.adblockplus.org/notification.json",
   suppress_first_run_page: false,
   disable_auto_updates: false,
   first_run_subscription_auto_select: true,
   notifications_ignoredcategories: [],
   allowed_connection_type: null

[Oleksandr, 2017/03/28 14:45:01]

I think it would be better to have an empty string here instead. 

allowed_connection_type: ""

This would remove the need for all the other changes file too. 

[sergei, 2017/03/28 14:46:35]

That where we started from, an empty string is a magic value.

 };
 
 // It is for non-objects, e.g. for string, number, etc.
+// Don't put here preferences used by adblockpluscore.
 let nullableValues_ExpectedTypes = {
   __proto__: null,
   allowed_connection_type: "string"
 };
 
 let preconfigurable = ["suppress_first_run_page", "disable_auto_updates",
   "first_run_subscription_auto_select", "allowed_connection_type"];
 
 let values;
 let path = _fileSystem.resolve("prefs.json");
@@ skipping 102 matching lines @@
   }
 
 // Define defaults
 for (let key in defaults)
   defineProperty(key);
 
 // Set values of prefs based on defaults
 values = Object.create(defaults);
 
 load();