@Test
public void testFillEmptyCells() {

  var rows = List.of(
      new Object[]{"a", "b"},
      new Object[]{null, "b1"},
      new Object[]{"a1", null}
  );
  var expected =  List.of(
      new Object[]{"a", "b"},
      new Object[]{"a", "b1"},
      new Object[]{"a1", "b1"}
  );
  CsvConverter.fillEmptyCells(rows);
  assertAll(
      IntStream.range(0, 3)
          .mapToObj(i -> () -> assertArrayEquals(expected.get(i), rows.get(i), "Row " + i))
  );
}