Fix EZP-23156: do not throw InvalidArgument exception on empty password

JIRA: https://jira.ez.no/browse/EZP-23156

Trying to login with an empty password will result in an uncaught `InvalidArgumentValue` exception.
This should not be the case, as it should still possible to (try to) authenticate with an empty password.

see also:
https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/MVC/Symfony/Security/Authentication/RepositoryAuthenticationProvider.php