You cannot control how they ask it. You cannot presume what context they are using. You cannot even presume that they are perfect in constructing the question even if you could know the context.

I agree, however I'm not presuming anything. I am, at least initially, trying to attribute a self-consistent set of beliefs to the person asking because this in general facilitates understanding among speakers.

learning to deal with that ambiguity makes me a better answerer.

But wouldn't you have saved yourself some time by asking the simple clarifying question, "Are you referring to the empty string in C?", which is a starting point justified by the wording of the question. If at this point they indicate a lack of understanding, then move forward in a different manner.