fix: do not try to fill gaps if there are no timeslots

program/services.py

@@ -847,6 +847,9 @@ def get_timerange_timeslot_entries(
     if not include_virtual:
         return [make_timeslot_entry(timeslot=timeslot) for timeslot in timeslots]
 
+    if timeslots is None:
+        return []
+
     timeslot_entries = []
     # gap before the first timeslot
     first_timeslot = timeslots.first()