MDL-47494 ddmarker: NOBUG return null for random_guess_score as it is incalculable

there are no shown drop zones and we don't know how many drag items have to be
dragged to the image. We could do the complicated calculation to find the area
of a polygon and compare it to the area of the image but it is an overly
complex a calculation I feel outside the scope of the current project.

question/type/ddmarker/question.php

@@ -363,6 +363,10 @@ class qtype_ddmarker_question extends qtype_ddtoimage_question_base {
         }
         return implode(', ', $goodhits);
     }
+
+    public function get_random_guess_score() {
+        return null;
+    }
 }
 
 /**

question/type/ddmarker/questiontype.php

@@ -359,4 +359,8 @@ class qtype_ddmarker extends qtype_ddtoimage_base {
         return $question;
     }
 
+    public function get_random_guess_score($questiondata) {
+        return null;
+    }
+
 }