Hi!
In Minetest 0.4.14, the singlenode map generator was removed from the world generation screen. The map generator is still there, but it can't be selected this way. You can only enable it indirectly by using advanced settings or with a mod or by some other way I am not aware of.
I wonder whether you think singlenode should be added back to the world generation screen.
For your information: The “singlenode” map generator is a simple map generator which creates (by default) a world only made out of air. Here's a screenshot.