ImmutableList<AnnotationMirror> nullableTypeAnnotations() {
    return nullableTypeAnnotation.map(ImmutableList::of).orElse(ImmutableList.of());
  }
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