Don't barf on air itemstacks found on disk

closes #5143

this is caused by bugs in PM4, where it saved air itemstacks when it wasn't supposed to.

These issues are now all addressed in PM5, since ItemSerializer won't accept air itemstacks.

src/item/Item.php

@@ -633,6 +633,9 @@ class Item implements \JsonSerializable{
 	 */
 	public static function nbtDeserialize(CompoundTag $tag) : Item{
 		$itemData = GlobalItemDataHandlers::getUpgrader()->upgradeItemStackNbt($tag);
+		if($itemData === null){
+			return VanillaItems::AIR();
+		}
 
 		try{
 			return GlobalItemDataHandlers::getDeserializer()->deserializeStack($itemData);