User time zone: selecting "Browser's time zone" option results in empty select

## Description

When creating or selecting a user you can select the browser time zone as the user time zone. The browser time zone is the default value.

When you select the value, the select becomes empty.


This is just a visual bug, when you save a user the browser time zone is still configured as their time zone.

You can also see an empty input when you clear the input. Since this also results in the user using the bowser time zone, we could also remove the option to clear the input via the x icon.

## Your Environment

* Graylog Version: 7.0

## Checklist
- [x] This issue fix need to be backported.
- [ ] Does this issue have **security** implications?