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 451 matching lines @@
     });
   },
 
   /**
    * Generator serializing filter data and yielding it line by line.
    */
   *exportData()
   {
     // Do not persist external subscriptions
     let subscriptions = this.subscriptions.filter(
-      s => !(s instanceof ExternalSubscription)
+      s => !(s instanceof ExternalSubscription) &&
+           !(s instanceof SpecialSubscription && s.filters.length == 0)
     );
 
     yield "# Adblock Plus preferences";
     yield "version=" + formatVersion;
 
     let saved = new Set();
     let buf = [];
 
     // Save subscriptions
     for (let subscription of subscriptions)
@@ skipping 290 matching lines @@
       else if (this.wantObj === false && val)
         this.curObj.push(val.replace(/\\\[/g, "["));
     }
     finally
     {
       Filter.knownFilters = origKnownFilters;
       Subscription.knownSubscriptions = origKnownSubscriptions;
     }
   }
 };