Update registration field if it is empty

Gijs Hendriksen
2019-11-14T13:59:51+01:00

Closes #101

Depends on concrexit!1353, 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.