docs: static Optional<String> nullableAnnotationFor(Element element, TypeMirror elementType) {
      List<? extends AnnotationMirror> typeAnnotations = elementType.getAnnotationMirrors();
      if (nullableAnnotationIndex(typeAnnotations).isPresent()) {
        return Optional.of("");
      }
      List<? extends AnnotationMirror> elementAnnotations = element.getAnnotationMirrors();
      OptionalInt nullableAnnotationIndex = nullableAnnotationIndex(elementAnnotations);
      if (nullableAnnotationIndex.isPresent()) {
        ImmutableList<String> annotations = annotationStrings(elementAnnotations);
        return Optional.of(annotations.get(nullableAnnotationIndex.getAsInt()) + " ");
      } else {
        return Optional.empty();
      }
    }