minecraft-src/data/minecraft/worldgen/placed_feature/grass_bonemeal.json

{
  "feature": "minecraft:single_piece_of_grass",
  "placement": [
    {
      "type": "minecraft:block_predicate_filter",
      "predicate": {
        "type": "minecraft:matching_blocks",
        "blocks": "minecraft:air"
      }
    }
  ]
}