[FIX] gamification: tolerate invalid definitions

If a goal definition with a target goal of 0 and "higher the better",
instead of dividing by 0, set a progress of 0% (it is not possible to
compute the progress)

Task id: 2057898

addons/gamification/models/challenge.py

@@ -743,7 +743,7 @@ class Challenge(models.Model):
                     all_reached = False
                 if goal.definition_condition == 'higher':
                     # can be over 100
-                    total_completeness += 100.0 * goal.current / goal.target_goal
+                    total_completeness += (100.0 * goal.current / goal.target_goal) if goal.target_goal else 0
                 elif goal.state == 'reached':
                     # for lower goals, can not get percentage so 0 or 100
                     total_completeness += 100

addons/gamification/models/goal.py

@@ -195,7 +195,7 @@ class Goal(models.Model):
                 if goal.current >= goal.target_goal:
                     goal.completeness = 100.0
                 else:
-                    goal.completeness = round(100.0 * goal.current / goal.target_goal, 2)
+                    goal.completeness = round(100.0 * goal.current / goal.target_goal, 2) if goal.target_goal else 0
             elif goal.current < goal.target_goal:
                 # a goal 'lower than' has only two values possible: 0 or 100%
                 goal.completeness = 100.0