If JAVA_HOME is not set, javadoc-plugin fails with NPE. See:
This bug is fixed in javadoc-plugin v. 3.0.1. Recently we use v. 3.0.0.
We can avoid this failure by upgrading the plugin to the latest version. (or by setting JAVA_HOME)
Used Maven and Java versions:
Apache Maven 3.5.2
Java version: 1.8.0_151