Comment 1 for bug 1446318

You are not talking about the tag browser but are instead talking about the "Manage Tags" dialog. In that dialog, when you set a tag to nothing you get a message "Item is blank: An item cannot be set to nothing. Delete it instead", which is not an "offer to delete". There is a "Delete" icon (the green recycle arrows) on that dialog that deletes the current tag, so there is a way to delete a tag in Manage Tags.

I agree that it shouldn't throw the error when you empty the value and it produces the "You can't do that" message. I will fix that. It won't delete the tag.