MDL-34226 multichoice qtype: correct is_complete_response for multian…

…swer

The multianswer qtype uses the multi-choice one, and the way it does its
responses array breaks an assumption that multi-choice was making.

question/type/multichoice/question.php

@@ -199,7 +199,7 @@ public function is_same_response(array $prevresponse, array $newresponse) {
     }
 
     public function is_complete_response(array $response) {
-        return array_key_exists('answer', $response);
+        return array_key_exists('answer', $response) && $response['answer'] !== '';
     }
 
     public function is_gradable_response(array $response) {