[BUG] [JAVA] Generated classes no longer contain instantiated lists

Created by: maloef

Some time ago, there was a change so that Java lists are instantiated instead of null:

private List<Foo> foos = new ArrayList<>();

instead of

private List<Foo> foos = null;

This was the change that provided instantiated lists: https://github.com/OpenAPITools/openapi-generator/pull/1683

It worked in version 4.0.0 and 4.1.0, but it no longer works in version 4.1.1 and 4.1.2.