Yea you got it. The doubler is 0 nic liquid.
I'm pretty new at this myself but I have found buying liquids with doublers from TV to be very cost effective and flexible.
Say you want 12mg nic. You can
buy the 36mg (1 oz) liquid and doubler (2 oz), if you mix it all you will get 3 oz of 12 mg.
So the cost at regular price is: 29 cents per ml. If you
buy the flavor of the week it goes down to 23 cents per ml. (plus $5 for shipping)
It also allows you to be flexible. I mix some 12mg, some 8mg and I vape the doubler to just pacify without nic.
There are other sites that do basically the same thing but I'm really enjoying the TV flavors so far.