private Optional<AnnotationMirror> visitAll(List<? extends TypeMirror> types) {
      return types.stream()
          .map(this::visit)
          .filter(Optional::isPresent)
          .findFirst()
          .orElse(Optional.empty());
    }
Returns a list that is either empty or contains a single element that is an appropriate
{@code @Nullable} type-annotation.
value/src/main/java/com/google/auto/value/processor/Nullables.java