this significantly reduces the size of schemas when state remaps are used (see pmmp/BedrockBlockUpgradeSchema@85b83b360e).
in addition, this will likely offer a substantial performance and memory saving when walls get flattened, which will eventually happen.
since PHP emits empty JSON objects as arrays, this makes it pretty annoying to work with the schemas in other languages. However, nullability is something most languages understand pretty easily.
This should continue to support old schemas.
