Fix UnifiedPush Server release build

[INFO] [ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:2.10.4:jar (attach-javadocs) on project unifiedpush-model-api: MavenReportException: Error while generating Javadoc:
[INFO] [ERROR] Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in http://docs.oracle.com/javase/8/docs/api/ are in the unnamed module.
[INFO] [ERROR]
[INFO] [ERROR] Command line was: /Library/Java/JavaVirtualMachines/openjdk-11.0.2.jdk/Contents/Home/bin/javadoc @options @packages