If no owner availability is given, assume avail for full event

If no availability is set for an owner then the current check thinks that the owner is not available at all. This is wrong as no default availability is set AFAIK.

This PR changes the checks s.t. if no availability is given, availability for the full event is assumed.

AKModel/tests/test_json_export.py

@@ -479,6 +479,10 @@ class JSONExportTest(TestCase):
 
                     for owner in slot.ak.owners.all():
                         # restricted owner availability
+                        if not owner.availabilities.all():
+                            # no availability for owner -> assume full event is covered
+                            continue
+
                         if not Availability.is_event_covered(
                             slot.event, owner.availabilities.all()
                         ):