Fix lessons_on_day to work correctly if a person has no lessons

(cherry picked from commit 6f774c1d)

CHANGELOG.rst

@@ -20,6 +20,7 @@ Fixed
 * Preference section verbose names were displayed in server language and not
   user language (fixed by using gettext_lazy).
 * Affected groups and persons in substitutions list were displayed multiple times.
+* ``lessons_on_day`` didn't work as expected if a person has no lessons.
 
 `2.0b2` - 2021-06-02
 --------------------

aleksis/apps/chronos/model_extensions.py

@@ -83,7 +83,11 @@ def lesson_periods_as_teacher(self):
 @Person.method
 def lessons_on_day(self, day: date):
     """Get all lessons of this person (either as participant or teacher) on the given day."""
-    return LessonPeriod.objects.on_day(day).filter_from_person(self).order_by("period__period")
+    qs = LessonPeriod.objects.on_day(day).filter_from_person(self)
+    if qs:
+        # This is a union queryset, so order by must be after the union.
+        return qs.order_by("period__period")
+    return None
 
 
 @Person.method