When creating a user, the time zone setting is not being persisted (Okta)

## Description

When you create a user `/system/users/new` and select a time zone like `New York`, it is not being persisted after creating the user. Instead the user has the time zone `UTC`. This happened to me on a system which implements Okta. It does not seem to occur on a system which uses no authentication provider. When setting a time zone while editing the user, it is being persisted.

## Your Environment

* Graylog Version: 6.0.0 rc2