Issue 5040 - fix saving of allowed_connection_type preference

src/FilterEngine.cpp

 /*
  * 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 467 matching lines @@
       this, callback, std::placeholders::_1));
 }
 
 void FilterEngine::RemoveFilterChangeCallback()
 {
   jsEngine->RemoveEventCallback("filterChange");
 }
 
 void FilterEngine::SetAllowedConnectionType(const std::string* value)
 {
-  SetPref("allowed_connection_type", value ? jsEngine->NewValue(*value) : nullptr);
+  SetPref("allowed_connection_type", value ? jsEngine->NewValue(*value) : jsEngine->NewValue(""));
 }
 
 std::unique_ptr<std::string> FilterEngine::GetAllowedConnectionType() const
 {
    auto prefValue = GetPref("allowed_connection_type");
-   if (prefValue->IsUndefined())
+   if (prefValue->AsString().empty())
      return nullptr;
    return std::unique_ptr<std::string>(new std::string(prefValue->AsString()));
 }
 
 void FilterEngine::FilterChanged(FilterEngine::FilterChangeCallback callback, JsValueList& params)
 {
   std::string action(params.size() >= 1 && !params[0]->IsNull() ? params[0]->AsString() : "");
   JsValuePtr item(params.size() >= 2 ? params[1] : jsEngine->NewValue(false));
   callback(action, item);
 }
@@ skipping 49 matching lines @@
     FilterPtr filter = GetWhitelistingFilter(currentUrl, contentTypeMask, parentUrl);
     if (filter)
     {
       return filter;
     }
     currentUrl = parentUrl;
   }
   while (urlIterator != documentUrls.end());
   return FilterPtr();
 }