World: account for null chunk edge case in tickChunk()

the target chunk may no longer be loaded if it was unloaded during a previous chunk's tick (e.g. during BlockGrowEvent).
Since the parent function iterates over a pre-selected array of chunks, the chunk will still be present in the list even if it's no longer loaded by the time it's reached.

src/world/World.php

@@ -1222,7 +1222,8 @@ class World implements ChunkManager{
 	private function tickChunk(int $chunkX, int $chunkZ) : void{
 		$chunk = $this->getChunk($chunkX, $chunkZ);
 		if($chunk === null){
-			throw new \InvalidArgumentException("Chunk is not loaded");
+			//the chunk may have been unloaded during a previous chunk's update (e.g. during BlockGrowEvent)
+			return;
 		}
 		foreach($this->getChunkEntities($chunkX, $chunkZ) as $entity){
 			$entity->onRandomUpdate();