droolsjbpm-knowledge build fails on JDK 11 due to Javadoc plugin

droolsjbpm-knowledge build fails with error on JDK 11:

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.0.1:javadoc (default-cli) on project assertj-core: An error has occurred in Javadoc report generation:
[ERROR] Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.