I believe if all the cards of a certain kind are gone from the deck and you are told to take that condition, you simply gain the benefit of not having to take the condition. What if you are offered the option to take a condition in exchange for some side effect and either you already have the condition or it is not present in the deck?
I would think you would not be able to gain the side effect because the wording "you may gain a _ condition to _" does not get fulfilled as you didn't gain the condition.