new Item(Item::AIR) instead of new Air() which cause errors

A E_RECOVERABLE_ERROR error happened: "Argument 1 passed to pocketmine\block\Generic::onBreak() must be an instance of pocketmine\item\Item, instance of pocketmine\block\Air given

AND

A E_RECOVERABLE_ERROR error happened: "Argument 1 passed to pocketmine\block\Block::getDrops() must be an instance of pocketmine\item\Item, instance of pocketmine\block\Air given

Fixed