Closes #101

Depends on concrexit!1353 (merged), as the API does not correctly accept empty field values.

Previous behaviour

If an event registration field is empty, the app does not send it to the website, meaning it is never updated.

Steps to reproduce:

1. Create an event with a registration field that is not required.
2. Register for the event and set the registration field.
3. Update your registration and clear the information field.
4. Notice the information field still contains the old value.

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 and set the registration field.
3. Update your registration and clear the information field.
4. Notice the information field is correctly updated.