What repository will return when object not found in tests

My question is : what repository will return when object not found in junit tests.

I have test like this :

@RunWith(MockitoJUnitRunner.class)
@SpringBootTest
public class CouponServiceTestSuite {
    private final static String HOME_TEAM = "home team";
    private final static String AWAY_TEAM = "away team";
    @Autowired
    private CouponService couponService;
    @MockBean
    private CouponRepository couponRepository;
    @MockBean
    private MatchRepository matchRepository;
    @Test
    public void shouldThrowException() {
        //Given
        //When
        //Then
        Assertions.assertThrows(BetWinnerException.class, () -> couponService.getCoupon(-6L));

I want to mock this like :

Mockito.when(couponRepository.findById(ArgumentMatchers.anyLong()).thenReturn(null);

My service class :

@Slf4j
@RequiredArgsConstructor
@Service
public class CouponService {
    private final CouponRepository couponRepository;
    private final MatchRepository matchRepository;
    private final CouponMapper couponMapper;

    public List<CouponDto> getCoupons() {
        log.debug("Getting all coupons");
        List<Coupon> couponList = couponRepository.findAll();
        List<CouponDto> couponDtoList = couponMapper.mapToCouponDtoList(couponList);
        log.debug("Return all coupons: {}", couponDtoList);
        return couponDtoList;
    }

    public CouponDto getCoupon(Long couponId) {
        log.debug("Getting coupon by id: {}", couponId);
        Coupon coupon = couponRepository.findById(couponId).orElseThrow(()
                -> new BetWinnerException(BetWinnerException.ERR_COUPON_NOT_FOUND_EXCEPTION));
        CouponDto couponDto = couponMapper.mapToCouponDto(coupon);
        log.debug("Return coupon: {}", couponDto);
        return couponDto;
    }

    public CouponDto createCoupon() {
        log.debug("Creating new coupon");
        Coupon coupon = couponRepository.save(new Coupon());
        CouponDto couponDto = couponMapper.mapToCouponDto(coupon);
        log.debug("Return created coupon: {}", couponDto);
        return couponDto;
    }

    public CouponDto addMatch(Long couponId, Long matchId) {
        log.debug("Add match to the coupon: {}{}", matchId, couponId);
        Coupon coupon = couponRepository.findById(couponId).orElseThrow(()
                -> new BetWinnerException(BetWinnerException.ERR_COUPON_NOT_FOUND_EXCEPTION));
        Match match = matchRepository.findById(matchId).orElseThrow(()
                -> new BetWinnerException(BetWinnerException.ERR_MATCH_NOT_FOUND_EXCEPTION));
        coupon.getMatchList().add(match);
        Coupon updatedCoupon = couponRepository.save(coupon);
        CouponDto couponDto = couponMapper.mapToCouponDto(updatedCoupon);
        log.debug("Return coupon with added match: {}", couponDto);
        return couponDto;
    }

    public boolean deleteCoupon(Long couponId) {
        log.debug("Deleting coupon id: {}", couponId);
        couponRepository.deleteById(couponId);
        if (couponRepository.existsById(couponId)) {
            log.debug("Coupon not deleted id: {}", couponId);
            return false;
        } else {
            log.debug("Coupon deleted id: {}", couponId);
            return true;
        }
    }
}

I thought that it returns null but when i do like this it returns NullPointerException. My service returns BetWinnerException when object is not found. So what it will return ? How should i create this test ?

Test like this works properly but i dont want to use id = -6. I just want to mock it somehow.

Thanks.