Allow null-values for additional cards

When null is given, there is no card to assign

There is currently no placeholder for the missing space.
This means, if a row has no card at some column it must also have no card
at any later column.

quiz.js

@@ -69,8 +69,10 @@ function shuffle(a) {
 function shuffle_cards(section) {
     let a = [];
     for (let i=0; i<cards_content[section].length; i++) {
+        if (cards_content[section][i] != null) {
             a.push(i);
         }
+    }
     shuffle(a);
     return a;
 }