[fix] fixed validation when register an user

no it's not... and that's unclear for the osiam operator (the one how install osiam ;) ). If the field is [configured](https://github.com/osiam/addon-self-administration/blob/master/src/main/deploy/addon-self-administration.properties#L51) is it also required? The operator do not has any chance to configure this. I know this is a little bit out of this PR, but we have to think about it. Perhaps we need something like [that](https://gist.github.com/dacrome/31edbb3b9e894258e937).