Chunks in the Nether can have skylight

The issue is related to incorrect chunk section fill, one of possible fixes is return 0 instead of skylight for client: https://github.com/paulevsGitch/BetterNetherBeta/blob/stapi-2.0/src/main/java/paulevs/bnb/mixin/common/FlattenedChunkMixin.java#L27-L37