QUESTION 1: In fact, is the value assigned by the system 0 (zero)?
QUESTION 2: If the question is sent without an answer, is the value assigned zero? That is, an unanswered question is equivalent to a question answered by marking an item of value "none"?
The motivation for the questions above is as follows. I didn't find an automatic way to add a neutral "I don't know" item to multiple choice questions. If the answer to question 2 is affirmative, it would be enough to tell the students to leave it blank.
the answer to question 2 is Yes. A question can be left blank and the score is 0 points. Of course, there are then different configurations to adjust these settings based on the scenario. For instance, you can configure the navigation so that selecting an answer is compulsory to unblock the next question (although there are some pedagogical caveats for this measure).
But if you don't need to make things complicated, the default quiz configuration is that students can skip a question without being penalized for that.
"A question can be left blank and the score is 0 points." - Is this the default behavior? If not what are the configuration settings to score 0 points?
Since you posted this to the Quiz forum, I expect you meant to say "Neutral quiz responses". Please note that the Moodle Questionnaire activity is something different from Moodle Quiz.
You might be interested in this discussion: https://moodle.org/mod/forum/discuss.php?d=411545#p1659737
The correct subject is "Neutral quiz responses".