Fix incorrect deadline in ModuleRevealState when it has no exercises

When ModuleRevealState has no exercises (that is, the corresponding
course module contains no exercises), then it must return
the module's deadline instead of `None` in the methods
`get_deadline()` and `get_latest_deadline()`.
`None` value prevents deadline-triggered reveal rules from working,
that is, the chapter model solution for the course module would
never be revealed when the deadline trigger is used.

In addition, this commit slightly optimizes ModuleRevealState.
In `get_latest_deadline()`, all exercises belong to the same
course module, thus their common deadlines are identical
(the module's deadline).

exercise/reveal_states.py

@@ -12,10 +12,12 @@
     from .cache.points import LearningObjectPoints, ModulePoints, ExercisePoints
 
 
-def _get_exercise_common_deadlines(exercise: ExercisePoints) -> List[datetime.datetime]:
-    deadlines = [exercise.closing_time]
-    if exercise.late_allowed and exercise.late_percent > 0:
-        deadlines.append(exercise.late_time)
+def _get_exercise_common_deadlines(
+        exercise_or_module: Union[ExercisePoints, ModulePoints],
+        ) -> List[datetime.datetime]:
+    deadlines = [exercise_or_module.closing_time]
+    if exercise_or_module.late_allowed and exercise_or_module.late_percent > 0:
+        deadlines.append(exercise_or_module.late_time)
     return deadlines
 
 
@@ -156,9 +158,8 @@ def get_latest_deadline(self) -> Optional[datetime.datetime]:
         deadlines = _get_exercise_common_deadlines(self.module)
         exercise_dict = {}
         for exercise in self.exercises:
-            deadlines.extend(_get_exercise_common_deadlines(exercise))
             exercise_dict[exercise.id] = exercise
-        if not self.max_deviation_fetched:
+        if not self.max_deviation_fetched and self.exercises:
             self.max_deviation = (
                 DeadlineRuleDeviation.objects
                 .filter(exercise__course_module_id=self.module_id)