Fix public events calendarjs API

Ghost User
2020-03-13T19:35:44+01:00

Closes #1033

Previous behaviour

Steps to reproduce:

1. Open the calendarjs API when not logged in using https://thalia.nu/api/v1/events/calendarjs/?start=2019-07-01T00:00:00&end=2020-07-01T00:00:00

2. Crash

New behaviour

Steps to validate that it works:

1. Open the calendarjs API when not logged in

2. Does not crash