500 error on apps.nextcloud.com account page via missing newsletter parameter leads to failed account updates

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### Summary
A POST request to https://apps.nextcloud.com/account/ returns an HTTP 500 Internal Server Error when the newsletter opt-out parameter ("I would like to receive app developer news and updates from Nextcloud by email") is omitted. Sending the same request with subscribe_to_news=off results in a normal HTTP 302 redirect and successful processing, indicating inconsistent backend handling of an optional consent parameter.

### Steps to reproduce

1. Log in to https://apps.nextcloud.com/account/
2. Update account details (e.g. first name, last name, email)
3. Leave "I would like to receive app developer news and updates from Nextcloud by email" unchecked
4. Submit the form

### Expected behaviour
Account updates should succeed regardless of newsletter opt-in or opt-out state.

### Actual behaviour
Omitting the subscribe_to_news parameter causes a server-side exception and HTTP 500 response.