That's actually why I looked this up in the first place. I think it's just air from what I could gather. The three sections of the pipe are laid on the seafloor like this (with the open end staying out of the water): https://www.wermac.org/nordstream/html_img66.html.

And then those three sections are welded together using an "air-tight habitat" surrounding them.

From what I've heard salt water getting into the pipe could make it really difficult to repair, and it might have to be scrapped altogether.