Fix unpublished events feed

Ghost Userrequested to merge
fix/unpublished-events into master

Previous behaviour

Steps to reproduce:

  1. 500 error when loading https://thalia.nu/api/v1/events/unpublished/?start=2018-12-14&end=2020-01-01

New behaviour

Steps to validate that it works:

  1. no error
Edited by Ghost User

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