scripts: point latest.json to nightly.json when there are no tags

In case this script is run as part of the pipeline of an user that forked the repository there may be no tags available. In this case, the script aborts in versions[-1], as there are no versions collected.

Since this situation is expected for MRs, then point the latest.json symlink to nightly.json instead, since it is the "latest" version available in such situation.

This does not affect the scheduled run of the web job that runs for the main repository, as in that case there are all the tags available.