I'm Dutch myself, and I can tell you that it's 'niks'. To answer your question, yes this is also a correct word to use in this situation. 'Niets' is the proper correct Dutch to say and write it, whereas 'niks' is the way we usually say it in real everyday life (unless you live in middle in the country, that's where 'niets' gets used more) :3
I think you can compare it British English and American English. It proper to speak the British way, but it's easier to speak the American way. I hope that made sense :)
Thank you for trying to learn the language, it's hard for many people, even Germans.