When the company gave your contract, they had the possibility to reduce the notice period that you had to give ('opzegtermijn van de werknemer') to 0 days. (Burgerlijk Wetboek Boek 7, artikel 672, 4 and 7, my summary, but I am not a lawyer: if nothing is written in the contract, it is one month, but the contract is allowed to change this.) They chose not to change it, so it is one month. Legally, you are covered. But that does not solve your problem, you still have the meeting.
From your description, it sounds you work 'gedetacheerd'. As in: all that your real employer does is find a project for you at another company. If there is no project for you, there is little meaningful work for you to do. There is perhaps not even a desk for you in your company's office.
And your previous project has ended. There is nothing left for you to do, and even worse: doing it may give legal problems because the NDA that you signed for that project probably does not allow this. You probably have no documentation to finish, no new people in the project to help get started. So don't consider that.
But you might still be able to do something. They might find a small project of one week for you. You can help with organizing the new year's drink. You can help looking at cv's of applicants. You can join a brainstorm session on which new clients your company could target. Realistically, this will not keep you busy for more than 3 days in this month, but be open for it. Don't plan any holiday or whatever during this month (unless you have enough free days remaining of course).
How to make this meeting smooth:
You can't. But your boss can, by being reasonable. Don't accept the month unpaid leave. Be clear about that, and don't give reasons if they don't ask. If they ask why, say that you don't do it because you have a contract for one more month. Repeat as many times as necessary.
Your boss most likely has already accepted that he has to pay for this month, but tries it anyway. ('Niet geschoten is altijd mis.')
Stay willing to work in this meeting: suggest to check cv's, do brainstorms or help with activities. Show you are cooperative, and your boss will become so too.
If there is nothing for you to do, he will not force you to come to office, because it will cost him a desk and chair. It's cheaper for him if you sit at your couch at home.
And yes, you will have a negative value to your boss this month. He will not like that. But that is not your problem, and probably, your company earned enough money off you in the previous months. Keep that in mind, if you worry about costing money.
And if your are really 'gedetacheerd': don't worry about references. First of all, in the Netherlands they are not so important as in the United States, and secondly, a new employer will not ask your 'detacheringsbureau' for a reference, but the company where you really did your work.
If your boss chooses to stand firm, and makes it a tough meeting: that's his choice. Show him that you will not change your mind, and he might stop.