Fix handling of empty answers in quizzes

Change-Id: Iebdace589553105383686a5b531c2968870932df

oioioi/quizzes/controllers.py

@@ -198,15 +198,15 @@ def _submit_answers(self, selected_answers, question, submission):
                 sub.save()
 
     def _get_selected_answers(self, form_data, field_id, question):
-        field_value = form_data.get(field_id)
+        field_value = form_data.get(field_id, '')
         if question.is_text_input:
             if isinstance(field_value, list):
                 return field_value[:1]
             return [field_value]
         elif question.is_multiple_choice:
-            return [int(a) for a in field_value]
+            return [int(a) for a in filter(bool,field_value)]
         else:
-            return [int(field_value)]
+            return [] if not field_value else [int(field_value)]
 
     def is_quiz_question_answer_case_ignored(self, question):
         return True