Username doesn't get revalidated if it gets overridden by E-Mail

# Settings

### Versions
* TYPO3 9.5.5
* femanager 5.0.0

### TypoScript

```
plugin.tx_femanager.settings.new.validation {
    username.required = 0
    username.uniqueInDb = 1
}
```

The initial unique check works fine, but if the username gets replaced by the email in `femanager/Classes/Controller/NewController.php` the username doesn't get revalidated.