Fix for <a href="https://bugzilla.novell.com/show_bug.cgi?id=381498.">https://bugzilla.novell.com/show_bug.cgi?id=381498.</a>;

Favicons were not loaded on the first attempt, when they had to be fetched by the favicon service, because the favicon service emits iconChanged with the requested URL if the favicon was in the cache, but with the hostname if the favicon had to be fetched.

Since Akregator keyed pending requests with the URL, newly fetched favicons were not recognised in FeedIconManager::slotIconChanged().

I don't know why the favicon service behaves so strangely, but I have changed Akregator to key all pending requests on hostname, so the service' response is always handled.

Also fixes a QString::isNull() problem caused by DBus not distinguishing between a null string and an empty string.