Redundant null check for JDK maps in generated code

[saarmbruster]

bc4493a introduces a redundant null check for JDK maps. I found this because Spotbugs complains abound loading a known null value. The generated code looks like the following example:

if (v == null) Objects.requireNonNull(v, "value for key: " + k);

Obviously, it makes no sense to call Objects.requireNonNull() on a value known to be null. The if (v == null) is new since the mentioned commit. I don't think this should be there. Without it the code looks fine to me.