@Test public void testCountingDistinct_duplicateEntries_withNulls() {

  assertThat(BiStream.of(1, null, 1, null).collect(BiCollectors.countingDistinct()))
      .isEqualTo(1);
  assertThat(BiStream.of(null, null, null, null).collect(BiCollectors.countingDistinct()))
      .isEqualTo(1);
  assertThat(BiStream.of(null, "one", null, "one").collect(BiCollectors.countingDistinct()))
      .isEqualTo(1);
}