McRegion: skip chunks with TerrainGenerated=false

legacy PM used to save even ungenerated chunks, and omitted some tags when doing so which we expect to always be present.

src/world/format/io/region/LegacyAnvilChunkTrait.php

@@ -51,7 +51,7 @@ trait LegacyAnvilChunkTrait{
 	/**
 	 * @throws CorruptedChunkException
 	 */
-	protected function deserializeChunk(string $data) : ChunkData{
+	protected function deserializeChunk(string $data) : ?ChunkData{
 		$decompressed = @zlib_decode($data);
 		if($decompressed === false){
 			throw new CorruptedChunkException("Failed to decompress chunk NBT");