Help: Fix compile for Qt5.5

Change-Id: I8b8063043d5a6be2dc8f9ce1e2e9e36646959050
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Robert Loehning <robert.loehning@qt.io>

src/plugins/help/helpviewer.cpp

@@ -109,7 +109,7 @@ QString HelpViewer::mimeFromUrl(const QUrl &url)
             return QLatin1String(e->mimeType);
         ++e;
     }
-    return "";
+    return QString();
 }
 
 bool HelpViewer::launchWithExternalApp(const QUrl &url)