[MC-195717] Custom Dimensions JSON Requires Seed

The bug

In the json file that defines custom dimensions, an error occurs if you do not provide a seed key. Expected behavior is for the game to use a random seed if none is provided via key.

This is the error that occurs if the seed key is removed:

No key seed in MapLike[{"type":"minecraft:noise","biome_source":{"type":"minecraft:multi_noise","seed":555,"biomes":[{"biome":"minecraft:soul_sand_valley","parameters":{"altitude":0,"temperature":0,"humidity":0,"weirdness":0.8,"offset":0}},{"biome":"minecraft:nether_wastes","parameters":{"altitude":0,"temperature":0,"humidity":0,"weirdness":0.8,"offset":0}}]},"settings":"minecraft:overworld"}]