When you have some items in your creative inventory go to "Survival Inventory" card and Shift-Click "Destroy Item". It should clear your inventory, but it doesn't, just nothing happens and you can use your items as usual (drop it too)

When you try to use /clear then, says No items were found on player

And when you add any new item to your inventory, you can use /clear again and it deletes all items you have.