Given a mandatory artifact link field in a tracker, if I create a new artifact by providing only the parent (a reverse link), the field must not be considered as empty.
Tuleap artifact links can be either direct or reverse. Having a link field as mandatory implies to have defined links. We have to take into account reverse links in this case.
In order to make things understandable from end user PoV, the "show reverse link" button should be removed and reverse links displayed by default. As it's a sensitive change, there should be a feature flag to fallback to the original behaviour in case of trouble.