Grades: fix ZeroDivisionError

lms/djangoapps/grades/subsection_grade.py

@@ -141,7 +141,10 @@ class NonZeroSubsectionGrade(SubsectionGradeBase):
 
     @property
     def percent_graded(self):
-        return self.graded_total.earned / self.graded_total.possible
+        if self.graded_total.possible > 0:
+            return self.graded_total.earned / self.graded_total.possible
+        else:
+            return 0.0
 
     @staticmethod
     def _compute_block_score(

lms/djangoapps/grades/tests/test_subsection_grade.py

@@ -36,3 +36,13 @@ class SubsectionGradeTest(GradeTestBase):
             read_grade.all_total.first_attempted = created_grade.all_total.first_attempted = None
             self.assertEqual(created_grade.all_total, read_grade.all_total)
             self.assertEqual(created_grade.percent_graded, 0.5)
+
+    def test_zero(self):
+        with mock_get_score(1, 0):
+            grade = CreateSubsectionGrade(
+                self.sequence,
+                self.course_structure,
+                self.subsection_grade_factory._submissions_scores,
+                self.subsection_grade_factory._csm_scores,
+            )
+            self.assertEqual(grade.percent_graded, 0.0)