Add exist/notExist predicated for EntityStream's Aggregations

[bsbodden]

You should be able to do:

  @Test
  void testExistPredicate() {
    List<Single<Integer>> releasedFilms = entityStream.of(Film.class) //
      .load(Film$.RELEASE_YEAR) //
      .filter(Film$.RELEASE_YEAR.exists()) //
      .reduce(ReducerFunction.COUNT).as("released") //
      .toList(Integer.class);
    assertThat(releasedFilms).hasSize(1);
    assertThat(releasedFilms.get(0).getFirst()).isEqualTo(2);
  }

  @Test
  void testNotExistPredicate() {
    List<Single<Integer>> releasedFilms = entityStream.of(Film.class) //
      .load(Film$.RELEASE_YEAR) //
      .filter(Film$.RELEASE_YEAR.notExists()) //
      .reduce(ReducerFunction.COUNT).as("released") //
      .toList(Integer.class);
    assertThat(releasedFilms).hasSize(1);
    assertThat(releasedFilms.get(0).getFirst()).isEqualTo(1);
  }