Allow empty values in event registrations API

Gijs Hendriksen
2019-09-15T22:01:43+02:00

The registration API does not allow empty values for text/integer fields that are not required.

Steps to reproduce:

1. Create an event with a registration field that is not required.
2. Register for the event.
3. Use the API to update your event, and leave the registration field empty.
4. The API does not allow it.

#### New behaviour
Steps to validate that it works:

1. Create an event with a registration field that is not required.
2. Register for the event.
3. Use the API to update your event, and leave the registration field empty.
4. The API allows it and your registration is successfully saved.