Testing My model mapper with spock and all test passes

Hello I have a problem with spock groovy testing. I need to write a Unit test for my model mapper, but always my test passes ok even I want to write wrong: This is My model mapper:

public class ModelMapper {
    public ConferenceRoomDto fromConferenceRoomToConferenceRoomDto(ConferenceRoom conferenceRoom) {
        return conferenceRoom == null ? null : ConferenceRoomDto.builder()
                .roomName(conferenceRoom.getRoomName())
                .description(conferenceRoom.getDescription())
                .projector(conferenceRoom.getProjector())
                .phoneNumber(conferenceRoom.getPhoneNumber())
                .numberOfSeats(conferenceRoom.getNumberOfSeats())
                .build();

And this is My test:

given:
    ConferenceRoom conferenceRoom1 = ConferenceRoom.builder()
            .roomName("r")
            .projector(true)
            .description("d")
            .phoneNumber("23")
            .build()
    when:
    modelMapper.fromConferenceRoomToConferenceRoomDto(conferenceRoom1)
    then:
    ConferenceRoomDto conferenceRoomDto1 = ConferenceRoomDto.builder()
            .build()
}

This tets should fail because I give values to my conference room and I expect dto without values but all test pass ok.