[Experimental World Gen] Generating Modified Vanilla Biomes in a Vanilla-Layered world produces "null" biomes.

When you modify biomes that appear in the Vanilla-Layered set (such as Plains) in custom terrain generation, it generates unmodified biomes that are set to "null".

I provided a datapack (bug-showcase.zip) to show the bug. It turns all would-be plains biomes in a Default world into "null".