At the moment, the best you can do is, when you get to the Terrain Generator select Empty. This will produce an Empty or blank terrain for you to work on. It's a flat, grey grid. However, it's still "terrain" in that you can actually switch to terrain editing mode and sculpt it, paint terrain textures on it, etc. I use Empty, ignore the grey grid, and build my interior levels there.
There have been requests on GitHub for a truly "Empty" level with absolutely nothing in it, but they've not added it yet.
Intel i9-10900K 5.10GHz, 64 GB RAM, Nvidia GTX 1080 ti 11GB, Windows 10 64-bit, dual monitor display
