Clang: Fix initial tidy settings tab

When no settings exist it has to be disabled.

Change-Id: I1cb8368edf182fa616930f5718323c8e03a0e649
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/cpptools/clangdiagnosticconfigswidget.cpp

@@ -307,6 +307,7 @@ void ClangDiagnosticConfigsWidget::syncClangTidyWidgets(const ClangDiagnosticCon
         break;
     }
 
+    m_tidyChecksWidget->setEnabled(!config.isReadOnly());
     connectClangTidyItemChanged();
 }