Sat4j cannot be built using maven 3.6.0
When compiling current code of Sat4j using maven 3.6.0, the following error occurs:
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.0.0-M1:jar (attach-javadocs) on project org.ow2.sat4j.core: Execution attach-javadocs of goal org.apache.maven.plugins:maven-javadoc-plugin:3.0.0-M1:jar failed.: NullPointerException -> [Help 1]