Add ability to skip uninitialized variables from being serialized

[null2264]

Currently, uninitialized variables are being serialized as null:

{
  "id": "minecraft:cobblestone",
  "weight": 60,
  "dimensions": null
}

But it'd be great if I skip it and generate a much cleaner JSON file:

{
  "id": "minecraft:cobblestone",
  "weight": 60
}

[falkreon]

May be worth it to tag this for the 1.2.x branch so this can be available before the Document update.

[Niko-sk2x]

I would like at least an option to actually serialize them as null, as I want all the values in my json to be provided explicitly