Test for anything but a specific item in a chest [duplicate]

I'm trying to test for anything but a specific item in a chest. I've heard that this would be possible by removing the brackets.

The normal to test for an item is;

/testforblock X Y Z minecraft:chest -1 {Items:[{id:"minecraft:diamond"}]}

And the syntax to test for anything but the item would be;

/testforblock X Y Z minecraft:chest -1 {Items:[id:"minecraft:diamond"]}

However, this doesn't work, probably because removing the brackets doesn't actually invert the command. Does anyone know how I can test for any item except a diamond in my chest?

The chest is emptied each time it is tested, so I'm testing for the following situations;

There's a diamond in the chest, there is an item that isn't a diamond in the chest, or there's nothing in the chest.