I also want to add that if you wanted to make this a singleplayer game, you'd have to use things like Display Block and Set Entity Hidden and the like, which often require a paid rank.
Game Value > Item Values > Inventory Items will act as a list of the players inventory. This will show up as a list of 36 "air" items if the inventory is empty. You can check this game value against a list of 36 air items to check if the player has an empty inventory.