public static <T> PCollections<T> pCollections() {
    return new PCollections<>();
  }
@Test
  @Category(ValidatesRunner.class)
  public void testFlattenPCollectionsEmptyThenParDo() {
    PCollection<String> output =
        PCollectionList.<String>empty(p)
            .apply(Flatten.pCollections())
            .setCoder(StringUtf8Coder.of())
            .apply(ParDo.of(new IdentityFn<>()));

    PAssert.that(output).empty();
    p.run();
  }