Clang: Fix showing option in tooltip for diagnostic from clang itself

Change-Id: I84972c7b206741e9040e4654e6bc031251aa478c
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangdiagnostictooltipwidget.cpp

@@ -289,7 +289,7 @@ private:
 
         const QString link = documentationUrlForOption(option);
         if (link.isEmpty())
-            return QString();
+            return option;
 
         return wrapInLink(option.toString(), link);
     }