When all of the items are removed from a shulker box, the item NBT tag remains despite it being unnecessary.

To reproduce:

1. Get a shulker box that you've never used before. You will notice it has no NBT tag. 
2. Place the shulker box.
3. Place an item in the shulker box.
4. Remove the item from the shulker box.
5. Break the shulker box. You will see that it has 1 NBT tag that it didn't before any items were placed in it.