Handle undefined topics

This permits to drop default list of topics without having a custom one.

assigned to @fcombernous

added breaking-change improvement labels

changed milestone to %2.0.0

created branch 11-handle-empty-topics to address this issue

mentioned in merge request !11 (merged)

changed title from Handle empty topics to Handle undefined topics

mentioned in commit 30401b58

closed with merge request !11 (merged)