Help: fix calling QDesktopServices::openUrl when opening about:blank

Workaround QTBUG-71833

Fixes: QTCREATORBUG-21515
Change-Id: Iffa363ab94a8f71d958b04932c9829887bb00dd7
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
(cherry picked from commit fc5caf3e0f)
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/help/helpviewer.cpp

@@ -138,6 +138,10 @@ bool HelpViewer::launchWithExternalApp(const QUrl &url)
         const QUrl &resolvedUrl = helpEngine.findFile(url);
         if (!resolvedUrl.isValid())
             return false;
+        // Workaround QTBUG-71833
+        // QHelpEngineCore::findFile returns a valid url even though the file does not exist
+        if (resolvedUrl.scheme() == "about" && resolvedUrl.path() == "blank")
+            return false;
 
         const QString& path = resolvedUrl.path();
         if (!canOpenPage(path)) {