…on new user registrations

This fixes a case where new user registration can become inadvertently disabled if:
- The `Banned usernames` field has a value
- The `Email must match` field is empty

This results in:
- The `fail2wp_admin_check_new_user` filter will be applied at l.422, because of the `Banned usernames` value
- The email match check starting at l.1039 will always be run regardless of the field value, and defaults to invalid
- The email match will always return invalid because there is no value to match against
- New user registrations are then effectively disabled, as all emails are found to be invalid