public static NullType asNullType(TypeMirror maybeNullType) {
    return maybeNullType.accept(NullTypeVisitor.INSTANCE, null);
  }
Returns a {@link NullType} if the {@link TypeMirror} represents the null type or throws an
{@link IllegalArgumentException}.
common/src/main/java/com/google/auto/common/MoreTypes.java