Fixed null answer in checkbox problem as incorrect.

TNL-765

common/lib/capa/capa/responsetypes.py

@@ -715,12 +715,13 @@ class ChoiceResponse(LoncapaResponse):
         if not isinstance(student_answer, list):
             student_answer = [student_answer]
 
+        no_empty_answer = student_answer != []
         student_answer = set(student_answer)
 
         required_selected = len(self.correct_choices - student_answer) == 0
         no_extra_selected = len(student_answer - self.correct_choices) == 0
 
-        correct = required_selected & no_extra_selected
+        correct = required_selected & no_extra_selected & no_empty_answer
 
         if correct:
             return CorrectMap(self.answer_id, 'correct')

common/lib/capa/capa/tests/test_responsetypes.py

@@ -1075,6 +1075,17 @@ class ChoiceResponseTest(ResponseTest):
         # No choice 3 exists --> mark incorrect
         self.assert_grade(problem, 'choice_3', 'incorrect')
 
+    def test_grade_with_no_checkbox_selected(self):
+        """
+        Test that answer marked as incorrect if no checkbox selected.
+        """
+        problem = self.build_problem(
+            choice_type='checkbox', choices=[False, False, False]
+        )
+
+        correct_map = problem.grade_answers({})
+        self.assertEqual(correct_map.get_correctness('1_2_1'), 'incorrect')
+
 
 class JavascriptResponseTest(ResponseTest):
     from capa.tests.response_xml_factory import JavascriptResponseXMLFactory