Role mapping filter is always reset when using in network activated mode

Because we don't return the roles early if not empty when filtered in `get_user_roles_from_sso()` they will always get clobbered by the `if` statement which sets a fresh empty array.

The function returns the `$network_roles` array if it is not `null`, but an empty array is not `null`.

https://github.com/humanmade/wp-simple-saml/blob/master/inc/namespace.php#L823