Template Manager: clicking on empty space deselect everything

Change-Id: I7cfbbd3a4f4283b9438a97eb383b460cad1f5a63

sfx2/source/control/thumbnailview.cxx

@@ -525,6 +525,8 @@ void ThumbnailView::MouseButtonDown( const MouseEvent& rMEvt )
 
             return;
         }
+        else if (!pItem)
+            deselectItems( );
     }
 
     Control::MouseButtonDown( rMEvt );