Find null returned from a Boolean method

Marks return null in a method declared to return java.lang.Boolean. Callers reasonably treat a Boolean as two-valued, and using the result in an if or a && unboxes it, so a null answer throws a NullPointerException at a place far from the method that produced it. A return null inside a lambda or a class body nested in the method is attributed to that body instead. A method annotated @Nullable has opted into three-valued logic and is not reported.