User should notice it. He pressed the `enter` key. He expects something to happen. And he will look to see what happened.
Perhaps we was meaning to make a new list item after the current list item, which happens to be empty. And that didn't work out as he had expected. Instead, it resulted in a new list item after the current list.
If that is the case, the user will simply press backspace and the changes will be exactly reversed.
And the user will learn that he is being prevented from creating two empty list items consecutively.
He will also learn that this is a way to create a new list item in the parent list, and if this was done in the middle of a list, he will learn that this is a means to split lists.
Over all, awesome.