Introduce optimistic default

[aosen-xiong]

In the following code,

class Unannotated {
  Object get() { return null; }
  void set(Object of) {}
}

@AnnotatedFor("nullness")
class AnnotatedUse {
  void use(Unannotated u) {
    u.get().toString();
    u.set(null);
  }
}

The usage of set will still produce a warning.

Introduce "optimistic" defaults for unannotated code - or a different mechanism, like using skipUses for uses of unannotated code.

Originally posted by @wmdietl in #1290 (review)