Chunks no longer exist in un-generated state

a non-generated chunk is now always represented by NULL. This forces the case of ungenerated chunks to be handled by all code, which is necessary because ungenerated chunks cannot be interacted with or modified in any meaningful way.

src/world/format/io/BaseWorldProvider.php

@@ -67,9 +67,6 @@ abstract class BaseWorldProvider implements WorldProvider{
 	}
 
 	public function saveChunk(int $chunkX, int $chunkZ, Chunk $chunk) : void{
-		if(!$chunk->isGenerated()){
-			throw new \InvalidStateException("Cannot save un-generated chunk");
-		}
 		$this->writeChunk($chunkX, $chunkZ, $chunk);
 	}