Issue 6857 - Do not serialize empty special subscriptions

test/filterStorage_readwrite.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 111 matching lines @@
     if (withEmptySpecial)
     {
       let specialSubscription =
         SpecialSubscription.createForFilter(Filter.fromText("!foo"));
       FilterStorage.addSubscription(specialSubscription);
 
       FilterStorage.removeFilter(Filter.fromText("!foo"), specialSubscription);
 
       test.equal(specialSubscription.filters.length, 0,
                  "No filters in special subscription");
-      test.ok([...FilterStorage.subscriptions()].indexOf(specialSubscription) != -1,
+      test.ok(new Set(FilterStorage.subscriptions()).has(specialSubscription),
               "Empty special subscription still in storage");
     }
 
     return FilterStorage.saveToDisk();
   }).then(() => testData).then(expected =>
   {
     test.deepEqual(canonize(IO._getFileContents(FilterStorage.sourceFile)),
                canonize(expected), "Read/write result");
   }).catch(unexpectedError.bind(test)).then(() => test.done());
 }
@@ skipping 120 matching lines @@
     return FilterStorage.restoreBackup(1);
   }).then(() =>
   {
     test.equal([...FilterStorage.subscriptions()][0].filters.length, 1, "Filter count after restoring backup");
     return FilterStorage.loadFromDisk();
   }).then(() =>
   {
     test.equal([...FilterStorage.subscriptions()][0].filters.length, 1, "Filter count after reloading");
   }).catch(unexpectedError.bind(test)).then(() => test.done());
 };