should support and empty rule set ("do nothing") configuration

[hgschmie]

It should be possible to "turn off" the default checks without having to specify any violations file. This would allow configuring the plugin in a stacked pom environment (having a base pom that wires up the plugin but does not impose any policy on a project) and the leave it to the actual projects to choose what checks it wants (e.g. turn on the default checks or supply a specific violations file).

[jamcole]

It appears you can use <skip>true</skip> or -Dmodernizer.skip=true in the sub-project to disable the checks even though it's not in the documentation; works for our situation.
Our parent pom contains <skip>${modernizer.skip}</skip> since the config setting takes precedence over the system property for some reason; this makes it work as expected so our sub-projects can disable the checks until they have a configuration specific to them.
Using that with maven's "combine.children" and "combine.self" config merging is how we manage all our parent pom config checks (we also use pmd in a similar manner).