reana-dev: git-aggregate-changelog always includes at least one version of each component

[mdonadoni]

The command git-aggregate-changelog is used to aggregate all the changelogs of all the REANA components into a single one for the final release.

However, it might happen that not all the components need a new release, so they should simply be skipped in the changelog. Right now, git-aggregate-changelog always includes at least one version for each REANA component, even when not needed.