fix: return null instead of ""

Closes #152

program/views.py

@@ -106,7 +106,7 @@ def timeslot_entry(*, timeslot: TimeSlot) -> dict:
         "id": timeslot.id,
         "playlistId": playlist_id,
         # `Timeslot.repetition_of` is a foreign key that can be null
-        "repetitionOfId": timeslot.repetition_of.id if timeslot.repetition_of else "",
+        "repetitionOfId": timeslot.repetition_of.id if timeslot.repetition_of else None,
         "scheduleDefaultPlaylistId": schedule.default_playlist_id,
         "scheduleId": schedule.id,
         "showCategories": ", ".join(show.category.values_list("name", flat=True)),