Issue 6857 - Do not serialize empty special subscriptions

lib/filterStorage.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present 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 429 matching lines @@
 
   /**
    * Generator serializing filter data and yielding it line by line.
    */
   *exportData()
   {
     // Do not persist external subscriptions
     let subscriptions = [];
     for (let subscription of this.subscriptions())
     {
-      if (!(subscription instanceof ExternalSubscription))
+      if (!(subscription instanceof ExternalSubscription) &&
+          !(subscription instanceof SpecialSubscription &&
+            subscription.filters.length == 0))
+      {
         subscriptions.push(subscription);
+      }
     }
 
     yield "# Adblock Plus preferences";
     yield "version=" + formatVersion;
 
     let saved = new Set();
     let buf = [];
 
     // Save subscriptions
     for (let subscription of subscriptions)
@@ skipping 188 matching lines @@
  * @param {Subscription} subscription filter subscription to be removed
  */
 function removeSubscriptionFilters(subscription)
 {
   if (!FilterStorage.knownSubscriptions.has(subscription.url))
     return;
 
   for (let filter of subscription.filters)
     filter.removeSubscription(subscription);
 }