There are many different credit scores. The range of valid FICO scores does not include zero, but internal acquisition and behavior scores that banks use might.
But in this case it's irrelevant because average balance is independent variable (input of the scoring model).
Credit scores don't work this way. Zero is not a valid credit score, and the things you mentioned describe different components of the score.
I work a lot with survey data, to be interpreted by humans. In this problem domain, zero is a sane value for an empty average, though "N/A" is usually better. I wish our language would let us define it as zero so long reports don't die in the middle. But it's about your problem domain, as usual.