Generating Javadoc for Java 9 compatible code

[Robert Scholte]

When generating with Javadoc with Java 11 or 12 for sources that are Java  
9 compatible I get the following output:

Loading source files for package com.foo...
Constructing Javadoc information...
javadoc: error - The code being documented uses modules but the packages  
defined in https://docs.oracle.com/javase/9/docs/api/ are in the unnamed  
module.
Standard Doclet version 12
Building tree for all the packages and classes...
Generating ....
:
:
:
1 error

This seems inconsistent: either don't create an error or don't generate  
the javadoc.
In this case there's probably a third option worth considering: create an  
element-list for the online Java 9 docs.
How should this be solved?

Based on the answer I can fix this for the next release of the  
maven-javadoc-plugin.

thanks,
Robert