MDL-64320 analytics: Validate discrete indicators separately

analytics/classes/local/indicator/binary.php

@@ -41,9 +41,7 @@ abstract class binary extends discrete {
      * @return array
      */
     public static final function get_classes() {
-        // It does not really matter, all \core_analytics\local\indicator\discrete get_classes calls have been overwriten as we
-        // only need 1 column here.
-        return array(0);
+        return [-1, 1];
     }
 
     /**