If no boxes were ticked, then the formula is not used.
However, you can use the NONEOF_TICKED variable, which equals 1 in the None of the above option was checked.

Thanks for the quick response.

We have a weird marking schema. For each question is is calculated as follows:
Correct option ticked: +1
Wrong option not ticked: +1
Correct option not ticket: -1
Wrong option ticket: -1
(We also need to count "None of the above" into account here")
Then the sum needs to be calculated and then scaled by the number of options (incl. "None of the above").

Do you have any recommendation on how to implement that? (In our exam, the "None of the above option" is only correct for exactly one question).

It seems to me very weird to treat the "None of the above" box the same way as other. The "None of the above" box is only here to make a difference between "the student did not answer" and "the student says no answer is correct".
Maybe the solution is to make a standard box with "None of the above" with something like that:

\begin{questionmult}{test}\scoring{b=1,m=-1}
 \AMCnoCompleteMulti
 Check correct answers.
 \begin{choices}
   \correctchoice{Correct A}
   \correctchoice{Correct B}
   \wrongchoice{Wrong}
   \lastchoices
   \wrongchoice{None of the above}
 \end{choices}
\end{questionmult}