Avoid sending empty tarballs/zips to Portico

[mauromsl]

When a journal publishes articles in a rolling basis, issues will be published ahead of time so they are displayed on the site. They are often also published with no articles, as editors want to use the issue page as a "call for papers" site.

Since the issues have a publication date, the Portico plugin will send them with no articles and flag them as "sent".

We have asked Portico how to handle issues that are always open for publication but, in the interim, we should add a check to make sure issues with no articles are ignored.