Skip update-responses update entries for versions without incremental or full update mar

While reviewing update-responses files for the 14.5 release, I noticed that an <update> entry for the latest 13.5.x release is still included in the linux update-responses files, although it is empty (it does not include any <patch URL="...">), since the legacy release is not available for Linux. This does not seem to affect the updater, but it would be more correct to not include this <update> entry at all.