I've seen several French bikes where the crown of the nut was 22.0mm yet the steertube ID was 22.2mm.
My Gitane was one of them. Since French threaded nuts can be hard to find, you may have no other option but to open up the top of the nut slightly. Be careful, it don't take much and if you do it right you will never be able to tell. Like was said above, first remove the nut and trial fit a 22.2mm stem, if it works, then all you need to do is open up the nut, if not, then your either sanding down a 22.2mm stem or hunting down a 22.0mm stem. Back in the day, SR made a 22.0 replacement to fit French bikes, that may be the most likely one to still be sitting on some shelf somewhere. Most of the collectors have gobbled up all the originals by now.
Sanding the stem don't take long, it's really not much to remove. I used to mask off the upper area with electrical tape to prevent scratching it by accident, then I'd wrap it with say 320 grit wet dry paper and sand it down till I got close to the size, then finish up with a much finer paper and repolish the stem smooth with some jewels polish.

Don't use a file, that just makes a mess and takes off far too much material. I've done dozens that way with no problems. To best give you an idea of how much .2mm is, the amount of aluminum you will have to sand off each side is about equal to the thickness of a standard sheet of Xerox paper.

I would also coat the new stem with grease to prevent it from ever getting stuck. I also do the same to seat posts as well.