public static string NullString(this Entity e)
{
    return e?.ToString();
}

public class ServiceRequest_Simple : IMapFrom<ServiceRequest>
{
    public string Room { get; set; }        //Room (Entity)
    public string Initiator { get; set; }   //User (Entity)

    public void Mapping(Profile profile)
    {
        profile.CreateMap<ServiceRequest, ServiceRequest_Simple>()
            .ForMember(dto => dto.Room, o => o.MapFrom(n => n.Room.NullString()))
            .ForMember(dto => dto.Initiator, o => o.MapFrom(n => n.Initiator.NullString()))
    }
}