Being stateless means not officially recognised as a citizen by any country in the world. Statelessness is not the same as nationality unknown. In fact, most foreigners whose nationality is unknown do have a nationality. They just do not have any documents to prove it. In the Netherlands, about 6,000 people are now registered as stateless in the Personal Records Database (BRP).
Statelessness in itself is not a ground for obtaining a residence permit. Stateless persons can obtain a residence permit just like other foreigners if they meet the conditions. Is there no country that wants to take back the stateless person? Then that person can obtain a special residence permit (no-fault permit).