Fix user registration behavior for external users when account creation is disabled

On a 8.10-SNAPSHOT:

• Install user registration addon
• Disable Allow New User Creation option
• Invite an external user and approve the request

=> The user receives an email and can define a password, but can't login afterwards
=> When looking at users and groups, the account was not created

=> Behavior is consistent but user should not be able to define a password in the first place since the option to allow new users is disabled